The Colorful Diversity of Thyroid Hormone Metabolites

in European Thyroid Journal
Author:
Josef Köhrle Institut für Experimentelle Endokrinologie, Charité Campus Virchow-Klinikum (CVK), Charité – Universitätsmedizin Berlin, corporate member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany

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*Josef Köhrle, Institut für Experimentelle Endokrinologie, Charité Campus Virchow-Klinikum (CVK), Charité – Universitätsmedizin Berlin, Augustenburger Platz 1, DE–13353 Berlin (Germany), E-Mail josef.koehrle@charite.de
DOI:
https://doi.org/10.1159/000497141
Volume/Issue:
Volume 8: Issue 3
Article Type:
Review Article
Page Range:
115–129
Online Publication Date:
21 May 2019
Copyright:
© 2019 European Thyroid Association Published by S. Karger AG, Basel 2019
Free access

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Since the discovery of L-thyroxine, the main secretory product of the thyroid gland, and its major metabolite T3, which exerts the majority of thyroid hormone action via ligand-dependent modulation of the function of T3 receptors in nuclei, mitochondria, and other subcellular compartments, various other T4-derived endogenous metabolites have been identified in blood and tissues of humans, animals, and early protochordates. This review addresses major historical milestones and experimental findings resulting in the discovery of the key enzymes of thyroid hormone metabolism, the three selenoprotein deiodinases, as well as the decarboxylases and amine oxidases involved in formation and degradation of recently identified endogenous thyroid hormone metabolites, i.e. 3-iodothyronamine and 3-thyroacetic acid. The concerted action of deiodinases 2 and 3 in regulation of local T3 availability is discussed. Special attention is given to the role of the thyromimetic “hot” metabolite 3,5-T2 and the “cool” 3-iodothyronamine, especially after administration of pharmacological doses of these endogenous thyroid hormone metabolites in various animal experimental models. In addition, available information on the biological roles of the two major acetic acid derivatives of thyroid hormones, i.e. Tetrac and Triac, as well as sulfated metabolites of thyroid hormones is reviewed. This review addresses the consequences of the existence of this broad spectrum of endogenous thyroid hormone metabolites, the “thyronome,” beyond the classical thyroid hormone profile comprising T4, T3, and rT3 for appropriate analytical coverage and clinical diagnostics using mass spectrometry versus immunoassays for determination of total and free concentrations of thyroid hormone metabolites in blood and tissues.

Abstract

Since the discovery of L-thyroxine, the main secretory product of the thyroid gland, and its major metabolite T3, which exerts the majority of thyroid hormone action via ligand-dependent modulation of the function of T3 receptors in nuclei, mitochondria, and other subcellular compartments, various other T4-derived endogenous metabolites have been identified in blood and tissues of humans, animals, and early protochordates. This review addresses major historical milestones and experimental findings resulting in the discovery of the key enzymes of thyroid hormone metabolism, the three selenoprotein deiodinases, as well as the decarboxylases and amine oxidases involved in formation and degradation of recently identified endogenous thyroid hormone metabolites, i.e. 3-iodothyronamine and 3-thyroacetic acid. The concerted action of deiodinases 2 and 3 in regulation of local T3 availability is discussed. Special attention is given to the role of the thyromimetic “hot” metabolite 3,5-T2 and the “cool” 3-iodothyronamine, especially after administration of pharmacological doses of these endogenous thyroid hormone metabolites in various animal experimental models. In addition, available information on the biological roles of the two major acetic acid derivatives of thyroid hormones, i.e. Tetrac and Triac, as well as sulfated metabolites of thyroid hormones is reviewed. This review addresses the consequences of the existence of this broad spectrum of endogenous thyroid hormone metabolites, the “thyronome,” beyond the classical thyroid hormone profile comprising T4, T3, and rT3 for appropriate analytical coverage and clinical diagnostics using mass spectrometry versus immunoassays for determination of total and free concentrations of thyroid hormone metabolites in blood and tissues.

Keywords: 3-iodothyronamine; 3,5-diiodothyronine; Tetraiodothyroacetic acid; Triiodothyroacetic acid; Reverse T3

Introduction

Thyroid extracts from sheep and larger animals have been successfully administered to relieve symptoms of hypothyroid myxedematous patients in the late 1890s. Observations made by mindful physicians experiencing with this novel therapeutic regimen clearly indicated increases in body temperature, pulse rate, and respiration as well as efficient reduction of myxedema and adipose tissue in treated patients [1]. Magnus-Levy, who fed thyroid extracts to a hypothyroid myxedematous patient in Frankfurt, Germany, provided first quantitative evaluation of this treatment observing increased oxygen consumption and CO2 production and increasing respiratory rate, a parameter, which over the following decades guided treating physicians to avoid thyroid hormone (TH) intoxication, tachycardia, and excessive body temperature [2].1

L-thyroxine (L-T4) is among the top 10 medically prescribed drugs worldwide, or ranking top in wealthy societies with affordable health systems. L-T4 is one of the safest and best-studied drugs currently available for treatment of hypothyroidism. Its wide and continuous use is not related to the need of effective and life-long treatment of congenital hypothyroidism, which is detected by worldwide screening of newborns in approximately 1 case per 2,500–3,000 live births. The major cause of L-T4 prescription is due to the development and subsequent medical treatment of autoimmune thyroiditis slowly but irreversibly impairing thyroid function and destroying the thyroid gland for still unknown reasons and by unclear pathogenic mechanisms. Thus, it comes as no surprise that among millions of mainly female patients treated for autoimmune thyroiditis during their reproductive age and later on a significant fraction of patients treated for hypothyroid symptoms (5–15%) report to be discontent with their L-T4 treatment prescribed by their physicians and search for alternative preparations [5, 6]. Most benign but also malignant thyroid diseases have a strong gender bias, affecting females more than men. While physicians are convinced that thyroid function tests of these patients and effects of T4 treatment are as expected, some patients still are not satisfied and report various complaints such as increases in body weight, appetite and food intake, as well as various presumably TH-related side effects including mood or behavioral changes. Considering the natural history of slowly developing autoimmune thyroiditis initially leading to mild and with time to more pronounced hypothyroidism, sometimes spanning over months and years without being diagnosed, both the physician and patient rarely have relevant information on thyroid status and thyroid function test before subclinical or manifest hypothyroidism develops. Thus, comparison and objective interpretation of successful L-T4 treatment remains quite difficult. Therefore, over the last years, the thyroid community discusses and experiments in more or less meaningful observational, interventional, prospective or retrospective, but rarely double-blind clinical studies whether such discontentment of patients (and sometimes physicians) with L-T4 monotherapy might be avoided, at least in a susceptible subgroup of patients by administration of more than L-T4 only, i.e. at least a combination of the “prohormone” together with a fraction of the truly thyromimetic L-T3 [5, 6]. T3 has been discovered in the 1950s [7, 8] and subsequently identified as the most relevant in vivo ligand for the two types of T3 receptors, i.e. TRα and TRβ, which mediate the majority of TH actions at the molecular, cellular, tissue as well as whole body level in humans and animals, taking advantage of their activation by the powerful iodinated TH T3 [9].

Online supplement 1 (for all online suppl. material, see online Supplementary Materials) gives a short summary of major achievements accomplished during the long history between discovery of the trace element iodine [10], its key role as major chemical constituent of TH T4 and 3,3′,5-L-triiodothyronine (T3) [11–13], the first characterization and clinical application of various animal-derived thyroid extracts, the establishment of reliable bioassays quantifying their action on oxygen consumption [14], basal metabolic rate, thermogenesis in experimental animals including tadpole metamorphosis [15], and culminating in the identification of receptors for T3, the major thyromimetic hormone [16–21].

This review discusses the biological function of endogenous metabolites, i.e. the “thyronome,” which are enzymatically generated from the parent TH T4. A focus is given to the thyromimetically active compounds 3,5-T2, Tetrac and Triac, as well as to the “cool” 3-T1-amine.

From the beginning of the discovery of TH, also various metabolites of iodothyronines conjugated with sulfate or glucuronic acid at the 4′OH position as well as oxidized or metabolized at their alanine amino acid side chain were reported, such as acetic acid and amine derivatives of TH with various iodination grades ranging from 0 to 4 [22]. Inconsistent data were reported with respect to their potency, mechanism, and mode of action, their occurrence in vivo and their physiological or pathophysiological relevance, not to mention here potential pharmacological administration. Consistently however, from the 1990s, biological actions and endogenous occurrence were also reported for 3,5-T2 [23], which, at lower concentrations appears to target mitochondria and to exert rapid and direct actions distinct from those of the classical T3 receptor binding ligand [24]. However, at higher concentrations, 3,5-T2 has been reported to suppress TSH and the HPT axis, to cause adverse cardiac effects similar to hyperthyroid conditions, and to regulate expression of T3-responsive target genes similar to T3 [25–27].

In 2004, a major discovery was made with the first identification of 3-iodothyronamine (3-T1AM) as a pharmacological agent with remarkable biological properties [28]. This iodine-poor biogenic amine synthesized, by ornithine decarboxylase (ODC) [29] and possibly other amino acid-metabolizing enzymes, reversibly reduced body temperature by 8°C in various animal models, exhibited negative inotropic and chronotropic effects on the heart, and if administered in close time relation, also prevented experimentally induced myocardial and brain infarct lesions [30]. However, quite high pharmacological doses were needed to exert these effects, which are currently under investigation. More than 100 years after the detection of the hormonal principle in the thyroid gland as iodinated amino acid derivatives [11, 31], successful attempts have been made to generate thyroids in a dish from embryonic stem cells and human-induced progenitor stem cells [32–35], which, as proof of principle, in a mouse model could restore TH in athyreotic hypothyroid mice. Thus, it appears realistic to expect treatment of hypothyroid diseases and congenital hypothyroidism by transplanting in vitro propagated thyroid follicles to hypothyroid patients during the next decade(s).

TH Deiodination and Related T4 Metabolites

The discovery of T3 as the main thyromimetically active hormone and its formation from its substituted precursor T4 in thyroidectomized patients opened the race for discovery and characterization of the enzymes catalyzing this and possibly related reactions generating a cascade of lower iodinated T4 metabolites with three, two, one, or none iodine left in the 5′,3′- and/or 5,3-position of the iodothyronine back bone (see Fig. 1a). Figure 1 provides an overview of further T4 metabolites handled by deiodination reactions, such as reverse-T3 (3,3′,5′-triiodo-L-thyronine, rT3), the two structural isomers 3,5-diiodo-L-thyronine (3,5-T2) and 3,3′-diiodo-L-thyronine (3,3′-T2), an 3-iodo-L-thyronine (3-T1). Deiodinases also accept the deaminated TH metabolites (THM) 3,3′,5,5′-tetraiodothyroacetic acid (Tetrac) and 3,3′,5-triiodothyroacetic acid (Triac) as well as the decarboxylated 3-T1AM as substrates [22, 30] (Fig. 1b; see details below). Apart from initial analyses using 131-iodine or 125-iodine radioactively labeled precursors and substrates, and paper- or liquid chromatography, the development of highly specific immunoassays for various THM created a wave of discoveries, not only of endogenous THM but also three different iodothyronine deiodinases (DIO1, -2, -3), the unrelated dehalogenase (DEHAL1) removing iodide from diiodotyrosine (DIT) and monoiodotyrosine (MIT), as well as several enzymes transferring activated sulfate residues to the 4′-hydroxy-group of iodothyronines in addition to UDP-glucuronosyltransferases transferring activated glucuronic acid to the 4′-hydroxy group [22]. Beyond the last two modification reactions, which are also related to and interfering with the metabolism of endogenous steroid hormones as well as exogenous drugs or xenobiotics, also TH metabolism at the side chain of iodothyronines was discovered, albeit enzymes involved in these reactions are still elusive, except of the recently identified ODC catalyzed formation of thyronamines from iodothyronine precursors [29]. Enzymes catalyzing oxidative decarboxylation/deamination to generate acetic acid derivatives such as Tetrac and Triac are not characterized in detail with respect to their cellular activity or specificity handling iodothyronine substrates. Whether propionic acid derivatives are endogenous THM generated by reductive deamination is not clear at this point [36]. While DIT and MIT are essential precursors generated during the thyroperoxidase-catalyzed TH biosynthesis on the synthesis and storage protein thyroglobulin produced and secreted into the colloid lumen by thyrocytes [37], at least DIT and possibly also MIT are also formed during oxidative degradation of TH in activated macrophages or leukocytes generating significant transient increases in DIT serum concentrations as analyzed by radioimmunoassays, for example in sepsis, severe inflammation, and related conditions [38] (see online suppl. Fig. S1). Formation of DIT (and possibly MIT) contributes to a rapid decrease in circulating TH under conditions of nonthyroidal illness [38, 39].

Fig. 1.
Fig. 1.

a Thyroid hormone metabolites and main metabolic pathways of T4, T3, and 3-T1AM. Deiodinases (DIO), ornithine decarboxylase (ODC), and amino-oxidases (AO) contribute to development-, tissue- and cell-specific metabolism of THM. T4, L-thyroxine; T3, 3,3′,5-triiodo-L-thyronine; 3-T1AM, 3-Iodothyronamine; DIO1, 2, type 1 and type 2 deiodinases. b Schematic view of TAM metabolism. T4 and its metabolites undergo a cascade of metabolic reactions which may lead to formation of acetic acid (TAC) and amine derivatives (TAM). Not all details of these reactions have been characterized. Organs symbolize major tissues studied and contributing to THM formation.

Citation: European Thyroid Journal 8, 3; 10.1159/000497141

The biochemical, molecular, and functional characterization of the enzymes catalyzing deiodination of TH and THM, initially based on immunoassay methods [40–43], posed a major still ongoing challenge. These membrane-integral enzymes cleaving the carbon-iodine bond are selenocysteine-containing proteins with a thioredoxin fold as structural motive, while their endogenous cofactor(s), in vivo regeneration, reaction mechanism, development- and cell-specific expression as well as individual regulation by (patho)physiological conditions and contribution to local and systemic T3 supply is still controversial (see online Supplement 2) [44–53]. Albeit significant effort has been made to biochemically characterize various monodeiodinase sequential reactions, the formal proof of formation of the thyromimetically active THM 3,5-T2 from its putative precursor T3 (Fig. 1a) in vitro has not been successful, while in vivo animal experimental and human data suggest that T3 might be the endogenous 3,5-T2 precursor [54–57]. This is of high interest because T4, T3, and 3,5-T2, to different extent, have been dubbed as “hot” TH [22, 25] (see online suppl. Fig. S2a), able to stimulate oxygen consumption, thermogenesis, intermediary metabolism, and various steps of lipid, carbohydrate and structural metabolism. In contrast, the reported action of 3-T1AM and its deiodination product T0AM (which is also found in vivo), at least at high pharmacological acute and chronic administration of these THM, decreases body temperature, oxygen consumption, and induces a shift from carbohydrate to lipid oxidation (the “cool” TH) (online suppl. Fig. S2b).

While biological functions have been assigned to 3,5-T2, the role of rT3, a major T4 metabolite generated by reductive deiodination at the tyrosyl ring of T4, either catalyzed by deiodinase 3 or deiodinase 1, is currently unclear [58]. Production, degradation, and serum concentration of the very short-lived rT3 are tightly controlled. Increases in rT3 serum concentrations have been found under various pathophysiological conditions [59, 60] (online Supplement 3). Tissue rT3 has recently been visualized in metamorphosing tadpoles (online suppl. Fig. S3), but its biological actions [61–67], if any at all, e.g. as T3-antagonist, remain as controversial2 as observations made for low-molecular-weight metabolites of other hormones interacting with nuclear receptors (e.g., seco-steroids, retinoids, or fatty acid-derived hormones) [22, 68–71].

rT3 represents one of the most enigmatic endogenous THM, already detected early after T3 as minor constituent in thyroglobulin [74, 75], but as an abundant T4 metabolite in human blood after development of chromatographic and immunoassay methods. rT3 concentrations in blood are equimolar or sometimes even higher than those of the active hormone T3 (Table 1) and typically changes are inverse to those of T3 [40, 41, 59, 72, 73, 85, 93]. rT3 is an avid substrate for both Dio1 and Dio2 [86] leading to formation of the inert 3,3′-diiodo-L-thyronine (3,3-T2) [22]. The in vitro observed competitive inhibition of Dio1 activity by rT3 probably has no physiological relevance [87]. Neither has its postulated role as a source of placental iodide supply to the fetus been confirmed [88], nor do stoichiometric considerations support a potential function of rT3 as an iodine source in phagocytosis-associated iodination of foreign (bacterial) proteins facilitated by activated deiodinases in monocytes or leucocytes [39, 89, 90]. Placental membranes abundantly express both sodium iodide symporter [91] and deiodinases [92–94].

Table 1.

Key characteristics of endogenous TH metabolites
Table 1.

The Concerted Actions of Dio2 and Dio3 Activities Regulate Local T3 Availability and Action

Type 3 deiodinase (Dio3) is the key enzyme in rT3 production, and current hypotheses support the fact that rT3 production via Dio3 prevents tissues and cells from inadequate exposure to the prohormone T4, which otherwise would possibly undergo 5′-deiodination to yield thyromimetically active T3 [85, 95, 96]. Regulation, upregulation, and increased activity of Dio3 turned out to be a major theme of developmental regulation of TH action as prevention of T3 production from T4 apparently favors cell proliferation and prevents cell differentiation induced by T3 formation and action. Such processes of enhanced expression of Dio3 either during development or neo-expression of Dio3 in pathophysiology identified this gene as a putative oncofetal gene, highly relevant in local regulation of proliferation of various cell types. High Dio3 concentrations were found in many tissues during early development including the brain, typically associated with high Dio3 activity supporting the concept of Dio3 function as an enzyme favoring proliferation. Compatible with that hypothesis is the observation that Dio3 is expressed in human-induced pluripotent stem cell-derived cardiomyocytes [97]. Currently, development of inhibitors of DIO3 is in progress, which might be of high value in the prevention of proliferation of tumor cells, which de novo express DIO3 and form rT3 [98]. Proof of principle of this concept has been provided by in vitro studies as well as animal experimental models, demonstrating that expression of Dio3 in various tumor models enhances proliferation while downregulation of Dio3 prevents proliferation and tumor growth [63, 64, 85, 96, 99–102]. Of interest in this context is the mirror-inverted regulation of Dio2 and Dio3, influencing cell cycle and survival of carcinoma cells as illustrated for basal cell carcinoma, colorectal cancer cells, and other tumor cells [64]. These observations raised the question on the regulation of Dio3 expression and thus production of rT3. One of the major factors involved might be hypoxia and the hypoxia-induced transcription factor HIF1α known to induce Dio3 expression apart from various growth factors and other signalling molecules [103]. This is also of clinical interest in the context of the syndrome of consumptive hypothyroidism, where overexpression of DIO3 in juvenile hemangioma leads to high DIO3 activity, which exceeds the production of T4 by the thyroid gland even if T4 is substituted. The activity of hemangioma DIO3 is sufficient to remove and inactivate all T4, leading to clinical hypothyroidism. So far, only removal of the tumor and thus DIO3 is an efficient treatment choice [104]. Alternatively, tissue transplantation might be an option. Also, for this condition, potent and selective DIO3 inhibitors would be valuable drugs.

Deaminated Acetic Acid Derivatives (Tetrac and Triac) Are Endogenous Biologically Active TH Metabolites

Soon after the discovery of the classical TH T4 and T3 as iodinated amino acid derivatives, formation of deaminated propionic, acetic acid, and formic acid derivatives has been demonstrated using chromatographic methods and radioiodine-labelled TH precursors as substrates [105–107]. This resulted in the detection of endogenous Tetrac and Triac as biologically active compounds (e.g., in goiter prevention assays), formation of these metabolites and their intermediates in various tissues (e.g., thyroid, liver, kidney, etc.) or their extracts and in subcellular fractions such as mitochondria and cytosols [7, 8, 36, 107–112] (for details see online Supplement 4).

Tetrac

Tetrac, the physiological T4 metabolite found in human serum in low nanomolar concentrations [82, 112, 113, 114], has recently received major attention as a powerful ligand of the cell membrane THM receptor ανβ3 integrin [115] and as a precursor for deiodination to Triac [116]. Both are short-lived and bind in serum to transthyretin [117, 118]. Triac bypasses MCT8, the main TH transmembrane transporter (THTT) in cellular uptake and passage through the blood-brain barrier, an observation leading to ongoing multicentric clinical trials to rescue neuronal developmental deficits in the AHDS syndrome caused by X-chromosomal MCT8 mutations [84, 119–123] (see below) based on observations in various experimental animal models. Clinical experience with Triac has been accumulated during its previous administration to ameliorate hyperthyroid symptoms and suppression of elevated TSH in TH resistance caused by mutated TRβ [84, 124–125, 135–137]. A recent report found elevated Tetrac concentrations in the sera of patients with Graves’ disease and release of Tetrac from stable isotope labeled T4 by various cell types including fibrocytes [138]. At the ανβ3 integrin THM receptor, TH initiate rapid signaling mediated by the MAPK/ERK cytosolic kinase cascades [115] but Tetrac efficiently competes for T4 and T3 effects, thus e.g. blocking their angiogenic effects in various (cellular) models of tumor proliferation, migration, tissue invasion, or differentiation [126–128].

Triac

Triac also was identified as primordial bioactive TH in the protochordate amphioxus, where Triac but not T3 is the bona fide deiodinase substrate [129] and active TR ligand in an early evolutionary context where the ancient glycoproteohormone “thyrostimulin,” a TSH precursor, regulates T4 synthesis [130, 131].

Triac has received major attention as a short-lived but potent T3-mimetic metabolite modulating expression of T3-responsive genes with some preference for TRβ binding including some mutated TRβ variants [124, 125, 132, 133]. Among those tissue selective targets is suppression of TSH in the pituitary, induction of spot 14 and DIOs in liver and other selected target tissues. Triac (and Tetrac) might not affect hypothalamic TRH [119] and cardiac function, while bone, skin, kidney, and liver endpoints and body weight parameters respond similarly to Triac but not identical to classical TH treatment (for review see [84]) explaining its abuse3 [137]. Recently, tissue selectivity has been linked to its ability to bypass MCT8 as THTT leading to its experimental use both in animal models of the AHDS syndrome and clinical trials [84, 119, 121, 122, 134] (for more details see online Supplement 4, Table 1, and review [84]). Whether endogenous Triac has relevance for cell-specific TH action in (patho)physiology, where altered serum concentrations have been reported [113], remains to be studied in more detail and with improved analytical tools such as mass spectrometry (MS), which can avoid limits of error-prone quantification of Triac crossreactivity in T3 immunoassays [22].

3,5-T2, a Neglected but Thyromimetically Active THM

3,5-T2 has been known and studied for a long time by researchers but has mainly been neglected by the community of clinically oriented thyroidologists. 3,5-T2 circulates in human serum, and during the 1980s, several immunoassays have been developed and applied to determine 3,5-T2 concentration in serum of healthy individuals and in various states of TH pathophysiology [76]. The concentration ranges reported then are surprisingly wide compared to that of the classical TH T4, T3, and rT3. Depending on the method, low to high nanomolar 3,5-T2 serum concentrations have been reported (for summary, see [78]). Main methodological issues in this context are high cross-reactivities of 3,5-T2 with several T3 antibodies used and, vice versa, T3 cross-reactivities in the 3,5-T2 immunoassays, as well as the difficulty in generating sufficiently pure tyrosyl ring radioactively labelled tracers for the radio-immunoassays applied then. Here, the main technical challenge is the avoidance of additional unwanted labelling of the phenolic-ring once two iodine atoms have been introduced into the 3- and 5-position of the tyrosyl ring. Recently, an immunoassay based on monoclonal antibodies has been developed and applied [78], and concentrations reported are quite low in the range of 0.15–0.7 nmol/L, with a remarkably wider concentration range than known for the classical TH. Furthermore, commercially available 3,5-T2 preparations are notoriously contaminated with traces of T3 (up to 2%), making it difficult to assess the biological activity of 3,5-T2 without interference by T3 contaminant activity.

While no reliable biological activities have been reported for the two other T2-isomers, 3,3′-T2 and 3′5′-T2, 3,5-T2 has been demonstrated to bind to T3 receptors [139] with quite high affinity and activity, and in vivo its administration led to suppression of TSH and altered expression of classical T3-regulated genes in liver and other tissues of animal experimental models [25, 26, 139, 140]. 3,5-T2 action on thyrotrope cell function has been demonstrated in various pituitary experimental models ex vivo, in situ, and also in pituitary cell lines, and relevant thyromimetic activity was observed focusing mainly on TSH suppression but also GH stimulation [27]. Interest of the TH research community in 3,5-T2 increased after the demonstration that 3,5-T2 exerts immediate, rapid action, probably mediated by activation of mitochondrial function but not by classical T3-like modulation of nuclear T3 receptor function (for reviews see [24, 54, 55, 57]). 3,5-T2 effects were shown to be independent of transcription and translation and thus interpreted as bona fide direct activities on intracellular compartments so far not in the focus of classical TH research. Current status and previous work have been extensively reviewed recently by researchers of the teams around Goglia, Lanni, and Moreno (see recent review [141]). Remarkable effects of 3,5-T2 in animal models have been shown with respect to rapid stimulation of oxygen consumption, antisteatotic effects in the liver, decrease in serum lipids, increased glucose consumption by various tissues including muscle, modulation of beta cell function as well as several effects on skeletal muscle including sirtuin signaling (Fig. 2a) [141, 142]. In a pilot volunteer study, two of the researchers administered themselves dose-escalating 3,5-T2 and observed weight reduction without side effects [143]. These observations have, however, not been confirmed. In contrast, a clinical study using a synthetic 3,5-T2 mimic (TRC150094) [144, 145] in a clinical study could not confirm such beneficial effects for this 3,5-T2 analogue compound, which was effective in rodent models. Most of the studies on 3,5-T2 have been performed in rats, a few mouse experiments have also been published [25, 140, 146]. The majority of experimental models employed started out with hypothyroid rats to whom 3,5-T2 was given, in part compared to classical T3. Recently, several studies have been performed in rodent models of high-fat diet-induced obesity, both in a preventive and a treatment approach either by coadministration of high-fat diet with 3,5-T2 or treatment of obese animals with 3,5-T2, but not all outcomes confirmed the antisteatotic effects in rats or humans [144, 147]. Concentrations needed to exert wanted biological effects seem to be quite high. In vitro, typically concentrations in the low micromolar range were used, and observations were made that 3,5-T2 concentrations higher than 10 μM already lead to detrimental effects on cells or tissues studied. In vivo, 3,5-T2 doses between 10 and 100 µg/100 g body weight were used, and administration forms were quite different (subcutaneously, i.p., etc), and mainly single doses were dispensed, while only few studies used chronic application up to 1 month [25, 26]. Apart from the beneficial, intended lipid-lowering and metabolism-enhancing effects, several studies in rats and mice observed that 3,5-T2 concentrations, which did not yet affect hepatic lipid status, already led to TSH suppression [25, 140] (Fig. 2b) and additionally to unwanted cardiac effects [26]. These observations showed clear dose-dependency, and long-term administration successfully suppressed TSH, T4, and T3 serum concentrations as well as hepatic T4 and T3 content while 3,5-T2 itself accumulated to unexpectedly high levels in mouse livers after repeated administration [25]. Mechanisms of cellular uptake, retention, and tissue accumulation of 3,5-T2 are not clear at this point. It is speculated that some tissues such as liver rapidly accumulate 3,5-T2 but metabolize it slowly or fail to eliminate it as conjugates or via deiodination. Apparently, 3,5-T2 is not released from the liver into the blood as typically observed for T3. Whether intrahepatic 3,5-T2 formation also occurs after administration of T3 doses is not yet known, and no increase in hepatic 3,5-T2 concentration was observed under comparable conditions in mice after T3 dosing [25]. The 3,5-T2 accumulation in the liver is somehow reminiscent of beneficial effects of T3 analogues such as eprotirome, which also was a beneficial antilipidemic agent accumulating in the liver by yet unknown mechanism [148, 149]. Effects of TH, their metabolites and analogues, on the liver with focus on hepatic lipid mechanism and induction of autophagy have recently been reviewed by Sinha et al. [150]. Cardiac mechanisms of action of 3,5-T2 have not been studied in detail. A recent publication by Sacripanti et al. [151] reported on application of relatively high concentrations of 3,5-T2 in the low micromolar range to rat cardiomyoblasts (H9C2 cells), and they also used isolated rat heart preparations perfused with 3,5-T2, T3, or T4. Authors observed 3,5-T2 uptake by cardiomyoblasts and slightly increased glucose consumption similar to previous observations in skeletal muscle by Moreno et al. [152]. However, in contrast to classical T3 action, no major 3,5-T2 effects were observed on chrono- or inotropic cardiac parameters. Cardiac tissue concentrations of 3,5-T2 are not yet established. Accorroni et al. [153] did not find 3,5-T2 in heart tissue, but Moreno [56] observed low femtomolar 3,5-T2 concentrations in rat liver (1.5 fmol/100 g tissue). Jonas et al. [25] analyzed 3,5-T2 concentrations in mouse liver before and after 3,5-T2 exposure and also reported femtomolar concentrations (limit of detection: around 5 nmol/g after administration of 0.25 µg/g body weight). In this setting, concentrations increased to 10–20 nmol/g, and after administration of 2.5 µg/g body weight to a range between 20–60 nmol/g.

Fig. 2.
Fig. 2.

a Effects of thyroid hormone metabolites 3,5-T2 and 3-T1AM. Overview of reported metabolic effects of the hot 3,5-T2 and the “cool” 3-T1AM on selected target tissues in experimental animals (see text for details). FA, fatty acids; NIS, sodium-iodide symporter of the thyroid; syst. aort. press., systolic aortic pressure. The figure was designed by Julika Lietzow, IEÉ. b 3,5-T2 effects on the HPT axis. 3,5-T2 suppresses the HPT axis in mice under HFD and standard diet. Already the low dose of 3,5-T2 (0.25 µg/g body weight) reduces hypothalamic Tshβ expression and T3 content in the liver and serum. Dio1, deiodinase type 1; HFD, high-fat diet; Trh, thyrotropin-releasing hormone; Tsh, thyrotropin; Tshr, TSH receptor; SD, standard diet. The figure was designed by Julika Lietzow, IEÉ [25].

Citation: European Thyroid Journal 8, 3; 10.1159/000497141

Strong evidence for direct action of 3,5-T2 after chronic treatment comes from observations that 3,5-T2 is effective in presence of deiodinase inhibitors PTU and iopanoic acid [55], while T3 actions are blocked by inhibition of 3,5-T2 production. However, several doses were needed to exert 3,5-T2 effects under these conditions [154]. 3,5-T2 is not only active in peripheral rat tissues such as liver, heart, or the pituitary but may also downregulate TRH expression as demonstrated by Padron et al. [26]. Apart from effects on hepatic lipid status and energy metabolism, 3,5-T2 also alters muscle composition from slow-twitch to fast-twitch muscles, which are characterized by glycolytic phenotype and increased phosphofructokinase expression [152]. Various studies examined underlying mechanisms involved in these changes and concluded that AMP kinase activation together with mitochondrial effects and increased GLUT4 expression in skeletal muscle lead to these effects, some of which might, however, require altered gene expression beyond direct rapid effects [152–155]. A comprehensive summary of 3,5-T2 effects in comparison to T3 and 3-T1AM is provided in Table 1 of the very recent review by Louzada and Carvalho [23]. While 3,5-T2 effects on endocrine pancreatic function remain ambiguous with respect to changes in blood insulin levels and insulin secretion, 3,5-T2 was also applied in an experimental model of diabetic nephropathy (streptozotocin-induced diabetes); however, concentrations used in this model were too high [156] and probably cannot be interpreted with respect to possible extrapolation on the clinically relevant conditions in humans.

Only limited data have been presented in the recent years on 3,5-T2 tissue concentrations. Pinna et al. [157] established and validated a method to determine T4 and T3 concentrations in rat brain using tissue homogenization in presence of Dio1 inhibitor PTU, solvent extraction, HPLC separation of various THM, and subsequent analysis of individual THM by specific radioimmunoassay in those extracts. They were the first to determine also concentrations of 3,5-T2, 3,3′-T2 and its sulfate 3,3′-T2-S in various brain regions and subfractions [157]. Several surprising results were observed. Apart from expected regional differences of THM in various rat brain regions including the hypothalamus and pituitary, the T4-to-T3 ratio was found as almost equimolar in contrast to circulating blood concentrations, where T4 exceeds T3 concentration by at least one magnitude. This indicates already that local control of T3 and THM production by DIO isoenzymes and cellular uptake/retention by THTT play a significant role in maintenance of steady-state levels and regulation of local (autonomic) T3 action. While T4 and T3 concentrations ranged between 1 and 6 pmol/g, those of rT3 and the T2 metabolites were 20- to 60-fold lower in the fmol/g concentration range, up to 50 fmol/g in the amygdala (half as high as liver), but pituitary content was below the limit of detection. 3,3′-T2 concentrations tended to be 2- to 3-fold higher as also observed in serum. Highest 3,3′-T2 concentrations were found in cerebellum (almost 200 fmol/g), while concentrations in the hypothalamus, pituitary, and liver were undetectable probably due to rapid export by LAT transporter [158, 159]. rT3 concentrations ranged around 50 fmol/g like in the liver with no major regional difference except 4-fold higher concentrations in septum. Surprisingly high and first time reported were 3,3-T2-S concentrations in brain regions (50–100 fmol/g), while the septum (200 fmol/g) and hypothalamus (ca 100 fmol/g) revealed higher concentrations, and both the pituitary and liver were negative. T3 sulfate was only detected in the hypothalamus (ca 70 fmol/g) and liver (150 fmol/g). The biological relevance of the THM and THM-sulfate concentrations remains open, but apparently the highly active local deiodinase activities (all three DIOs including DIO1 were measured) in the brain, hypothalamus, and pituitary generate a region-specific metabolite profile strongly influenced by local expression of various THTT. While 3,3-T2 and its sulfate may represent end products of TH metabolism undergoing elimination, the potential biological function of 3,5-T2 as “thyromimetic” agent and of T3-sulfate as a local reservoir of T3, which can be regenerated to active T3 by locally active sulfatases, remains to be studied in more detail. Pinna et al. [157] also presented detailed analysis of expression of Dio enzyme activities in these brain regions and observed clear and distinct region-specific circadian patterns of T4, T3 and 3,5-T2 concentrations in the midbrain, cerebellum, and liver. In other publications, these authors demonstrated that brain DIO activities and TH contents are influenced by various nutritional (patho)physiological conditions and pharmaceutical drugs [160–162].

3,5-T2 Serum Concentrations

Application of the monoclonal antibody-based chemiluminescence immunoassay for 3,5-T2, which shows no relevant cross-reactivity with known naturally occurring TH metabolites in human serum, revealed serum concentrations in healthy individuals around 0.2 nM or higher. Surprisingly, no relevant, significant changes were observed in hyper- (n = 24) or hypothyroid (n = 31) patients with clearly altered T4 and T3 concentrations [78]. However, thyroid cancer patients (n = 100) after thyroidectomy and/or radioiodine treatment, substituted with T4 to euthyroidism, showed higher serum concentrations around 0.45 nM compared to healthy controls (n = 99). No differences were observed between males and females or depending on age. Authors assumed that patients on oral L-T4, lacking endogenous functional thyroid tissue, might generate higher 3,5-T2 concentrations during the absorption process of L-T4 in the gastrointestinal mucosa, similar to elevated 3-T1AM concentration found in the same patient group on oral L-T4 [81]. A similar study on n = 143 patients with thyroid cancer on T4 monotherapy revealed no correlation of 3,5-T2 concentrations (around 0.65 nM) with TSH, T4, or T3. T3 did not correlate with 3,5-T2 concentration, also no correlation to quality of life and 3,5-T2 concentration was observed [80]. Elevated 3,5-T2 concentrations were found in patients with nonthyroidal illness in cardiac context after postoperative atrial fibrillation. Here, 3,5-T2 serum concentrations directly correlated with reverse-T3 concentration in patients with nonthyroidal illness, and an inverse correlation was observed for preoperative fT3 [163]. Considering that endogenous pathway(s) of formation of 3,5-T2 are still unclear, the association between nonthyroidal illness and elevated 3,5-T2 might indicate enhanced degradation of T3 to 3,5-T2 or decreased 3,5-T2 degradation. The studies do not yet allow to distinguish between these two hypotheses. 3,5-T2 serum concentrations were also elevated in critically ill patients on intensive care compared to healthy volunteers, and especially in non-survivors, higher 3,5-T2 concentrations up to 10 nmol/L were observed [164]. Apparently, elevated 3,5-T2 observed in few individuals of an otherwise healthy, epidemiological anonymized study population may indicate underlying yet unknown disease(s) [79]. The reference range of this healthy population located in Pomerania (761 euthyroid participants) covered a median 3,5-T2 serum concentration of about 0.24 nM, and a significant portion (up to 1/3) of this population had 3,5-T2 concentrations at or below the limit of detection of the assay [79]. In this study, associations between 3,5-T2 were found with TSH and leptin as well as fasting serum glucose but not TH status. More detailed analyses of 3,5-T2 concentrations in serum together with urine metabolites of healthy individuals of the SHIP-TREND study [165] revealed surprising 3,5-T2 associations with trigonelline, pyroglutamate, acetone, and hippurate concentrations. These urine metabolites represent a “signature of coffee users”; however, a mechanistic link between elevated 3,5-T2 concentrations and these metabolites has not yet been established. Probably, omics technologies applied to human blood and urine in combination with transcriptomics and proteomics from animal experiments might unravel links between 3,5-T2 metabolism and other pathways relevant to THM homeostasis [165–167]. Further experimental and clinical studies are needed to shed more light on the biological activity of 3,5-T2 in energy homeostasis and metabolism.

Of major interest in this context are observations made in nonmammalian species, such as some fish species [168]. Two forms of TRβ T3 receptors have been identified in their genome. A long TRβ isoform with a 9-amino-acid insert at the beginning of the ligand-binding domain, and a shorter version without this insert. Interestingly, 3,5-T2 binds and activates the long TRβ isoforms, while T3 is selective for the short TRβ isoform activation. Metabolic and regulatory impact of these observations needs further studies, and so far, no such different TRβ isoforms have been identified in humans or mammals [168, 169].

Pinna et al. [170] reported elevated 3,5-T2 serum concentration in sepsis patients (47 pmol/L), higher than under several other conditions of illness. They also analyzed tissue concentrations of 3,5-T2 and found in normal human brain tissues 70–150 fmol/g, which were by a factor of 20 lower than those observed for T3. They also compared 3,5-T2 concentrations in the brain, pituitary, and liver of rats. Some tissues had detectable 3,5-T2 concentrations in the range of 20–50 fmol/g, while the liver ranked at the highest concentrations with almost 100 fmol/g. Thus, in their rat study, 3,5-T2 concentrations were similar to those of reverse-T3 in most regions, while 3,3′-T2 concentrations were higher. All the analyses have been performed with immunoassays [157]. They even observed circadian variations with highest values around 60 fmol/g during the light phase around noon. Iannucci et al. [171] compared effects of 3,5-T2 and T3 in a rat high-fat diet model using metabolomics analysis. They reported an induction of autophagy and anti-steatotic effects in the liver, but also observed that 3,5-T2, but not T3, would counteract impaired signalling via the AKT and MAPK/ERK pathways disturbed by high-fat diet. They compared a 10-fold lower T3 dose to 25 µg/100 g body weight of 3,5-T2 injected i.p. Unfortunately, no control diet group was reported. Hormone application was performed for 1 week daily. In summary, several in vitro and in vivo observations in rodent and human context indicate a relevant (patho)physiological role for the THM 3,5-T2, which is distinct from actions and changes in tissue and serum concentrations of classical TH T4, T3, rT3 as well as serum TSH. However, further studies will be needed to unravel the relevance of these observations for THM action and clinical practice.

Particular Features and Pharmacological Actions of the Endogenous THM 3-T1AM

The 2004 discovery of transient hypothermia effects of 3-T1AM after injection of rodents by the group of Scanlan [28] raised a wave of studies addressing biosynthesis of this aminergic metabolite of TH, its concentrations in animal models and humans under various pathophysiological conditions and opened the search for mechanism of action and other effects of this peculiar aminergic THM. So far, the majority of studies has been conducted with application of pharmacologically rather high 3-T1AM doses, and recent observations with repeated administration or lower doses did not confirm all hypothermic or metabolic effects initially observed [172–174]. Thyronamine metabolites of TH were already observed early after discovery of thyroxine and T3 (for recent reviews see [22, 30, 175]). However, these studies in rodents and other animal experiments did not yet lead to a clinical application or routine analysis of 3-T1AM serum concentrations. Analysis of autoradiographic images after application of radiolabeled T4, T3, and reverse-T3 raised the hypothesis that TH, similar to other amino acids, might be precursors of aminergic metabolites similar to catecholamines, serotonin, or dopamine [176]. Dratman [176] proposed aromatic amino acid decarboxylase (AADC) as potential enzyme generating thyronamines, based on her observations of accumulation of radioactivity in synaptic vesicles and nerve endings. Development of a monoclonal-based immunoassay for 3-T1AM allowed first immunological measurements of thyronamines in blood of humans and rodents detecting rather unexpectedly high 3-T1AM concentrations in the range of those of T3 or even higher [81]. The observation that 3-T1AM concentrations were higher in thyroidectomized thyroid cancer patients on T4 supplementation (similar to elevated 3,5-T2 concentrations, see above) compared to controls, raised the possibility that 3-T1AM might be formed during oral resorption of T4 or other THM. This hypothesis was tested using everted mouse intestinal sacs incubated with T4 and other TH, and indeed, 3,5-T2 and 3-T1AM formation could be detected after incubation with T4, using LC-MS analytics [29]. Furthermore, the hypothesis was tested whether aromatic AADC can form thyronamines in vitro. This was not the case using human recombinant AADC and TH as potential substrates, and also analysis of 3-T1AM concentrations in patients lacking AADC showed comparable 3-T1AM concentrations as in healthy individuals, excluding AADC as source of thyronamine production [177]. In contrast, another decarboxylase, ODC, efficiently generated thyronamines including 3-T1AM from TH precursors, making this enzyme a likely candidate of biosynthesis of thyronamines [29] not excluding potential other decarboxylases involved in their production (Fig. 1b).

Human serum concentrations of 3-T1AM are in the range of 10–70 nM as determined by chemiluminescence immune assay [81]. No major differences were observed between males and females, and no age-dependent changes have yet been identified. In human serum, 3-T1AM has a remarkably long half-life. Even 6 days after T4 withdrawal, no decrease in concentrations was observed, while T4 and T3 concentrations decreased as expected [81]. Authors explained this remarkable stability for a biogenic amine with its high-affinity binding to serum apolipoprotein B100, which has been identified as highly specific and high-affinity binding protein for 3-T1AM by the Scanlan group [178]. Serum 3-T1AM concentration concentrations did not decrease in nonthyroidal illness or postoperative atrial fibrillation [163], while lower concentrations were observed in critically ill ICU patients compared to healthy individuals [164]. However, ICU survivors and non-survivors did not differ in their serum 3-T1AM concentrations.

A series of metabolic and energy metabolism effects was observed for 3-T1AM. Apart from transitory hypothermia, cardiac function was affected by bradycardia and decreased output, heart rate and contractility were reduced [28, 179–184]. 3-T1AM application at these doses decreased metabolic rate, shifted metabolism from carbohydrate to lipids and was accompanied by elevated H2O2 production [185–188]. Again, sirtuin expression has been reported to be affected by pharmacological daily doses of 10 and 25 mg/kg 3-T1AM, and SIRT4- and SIRT6-dependent genes responded in the liver after 3-T1AM administration in subchronic protocols using female CD1 mice [188]. Analysis of 3-T1AM concentration in various tissues revealed its accumulation in a dose-dependent manner in the liver, white adipose tissue, muscle, and heart [188]. Basal concentrations differed between these tissues, with heart and muscle leading in the range of 18–20 pmol/g, liver at 7.7 pmol/g, and white adipose tissue at a low 0.5 pmol/g. Remarkably, 3-T1AM concentrations affect insulin secretion and blood glucose in various models [181, 182, 189–191]. Reduced insulin secretion has been accompanied by hyperglycemia, but in another study no changes in blood glucose and glucose tolerance were observed [190]. These different outcomes might depend on different application forms and doses of 3-T1AM used as well as in vivo versus in vitro studies using isolated pancreatic islets [181, 189, 191]. 3-T1AM affects mitochondrial energy machinery already at nanomolar concentrations and via decreasing the cellular ATP/ADP ratio impairs glucose-stimulated insulin secretion in vitro using murine MIN6 pancreatic β-cells as model. The major 3-T1AM metabolite 3-T1 acetic acid (TA1) can mimic part of these effects albeit with lower potency [191]. These experimental observations support previous in vivo effects of 3-T1AM achieved with pharmacological 3-T1AM concentrations [181, 182, 190, 192], while lower doses did not result in the same outcome [172].

The mechanisms involved in various 3-T1AM actions are still controversial and include several types of membrane receptors and intracellular targets (e.g., mitochondria) apart from the initially identified receptor trace amine-associated receptor TAAR1 (e.g., ADRA2A, TRPM8, TAAR5, TAAR8, etc.) (for recent reviews see [30, 175, 190, 193–196]). Besides activating TAAR1, also adrenergic receptor signaling has been documented as well as induction of biased signaling on serotonin 1b receptor [197]. Whether 3-T1AM has potential therapeutic applications in protecting from cardiac- or neuroischemia requires further studies [180, 184]. Recently, also central effects of 3-T1AM were observed, which led to vasodilation in tails of mice [173] as well as various behavioral and pharmacological effects in the brain, which require further analysis especially with respect to the question whether 3-T1AM or its rapidly formed metabolite TA1 is the mediator of those actions (for recent detailed reviews see [198, 199]). Glossmann and Lutz [200] recently even proposed that 3-T1AM might be formed by microbiota during gastrointestinal passage of TH. However, no experimental evidence or data were provided to support this assumption. Several reviews comprehensively and in detail summarized and discussed observations made on 3-T1AM, synthesis function, and pharmacological action [30, 153, 199]. Apart from its endocrine relevant actions, 3-T1AM is a powerful ligand and modulator of Transient Receptor Potential Melastatin 8 (TRPM8) [194, 201, 202] with higher potency exerted by this endogenous multitarget activating ligand 3-T1AM than that of established TRPM8 ligands such as icilin or menthol activating this “cold receptor.” TRPM8 activation blunts function of the Ca-channel Transient Receptor Potential Vanilloid (TRPV1), which is activated by heat, capsaicin and expressed on many tumor cells and leading to activation and secretion of growth factors and cytokines [202, 203]. Thus, 3-T1AM might be a powerful agent to inhibit such adverse effects exerted by TRPV1.

Open Issues in THM Research

Apart from the open questions on site(s) of production of TAM and enzymes involved in its biosynthesis apart from DIOs and ODC (see above) the exact reaction mechanisms of the three DIO isoenzymes is still not clear [46, 47, 51, 204–206], even if recently major progress has been made to clarify the exact role of essential selenocysteines in this catalytic reductive cleavage of the aromatic carbon-iodine bond as well as the preference for tyrosyl versus phenolic ring deiodination. A first X-ray structural analysis for the sulfur-homologue of DIO3 has been published [47] and interesting small-molecule selenium-based mimics for DIO enzymes have been developed which allow insight into the mechanisms of reaction [45, 49, 204]. Currently, the exact nature of the required endogenous reducing cofactor(s) of DIO reactions is elusive, and it remains to be studied in detail which of the THM so far identified in vivo or ex vivo are substrates of which THM metabolizing enzymes and in which cell type or tissues such reactions are prevailing. Sulfated THM have been described by many groups, but their exact metabolic fate, potential targets mediating their biological action, and the (patho)physiological relevance of altered THM sulfate concentrations in blood remains to be elucidated in more detail. 4′-OH sulfation of THM creates substrates selectively handled by DIO1 [207–213], and especially T3 sulfate might represent a reservoir of active T3. TH sulfates undergo enterohepatic recycling, and a sulfated T2 metabolite (“compound W”) [43, 94, 214–216] might represent a valuable biomarker for fetal thyroid function secreted across the placental membrane into the maternal circulation (for details see online Supplement 5).

Impact of Colorful TH Metabolite Diversity on Analytical Challenges in Clinical Practice and Research

Thyroid function tests currently focus on the determination of TSH by immunoassay-based methods as first-line biomarker. Depending on elevation or suppression of TSH, more detailed analyses are indicated, and measurement of TSH is repeated and complemented with determination of free T4 (fT4) serum concentrations, occasionally also fT3 is determined. In pediatric clinical praxis, preference is frequently given to determination of total T4 considering age- and development-dependent changes in reference ranges as well as distrust in readouts of serum free TH concentration in pediatric patients. Also, during pregnancy special attention has to be given to trimester-specific reference ranges for TH function tests, which are related to increased TBG production and secretion during pregnancy or associated with oral contraception [217–220].

Figure 3 illustrates quantitative relationships between the prohormone T4 and the major THM discussed in this article. This picture intends to visualize the analytical problems generated by the vast excess of total T4 concentration in blood (58–161 nM), compared to total T3 (<2.8 nM) and concentrations of total 3,5-T2 (0.5 nM) as well as total 3-T1AM determined by immunoassay (<30 nM) (see also Table 1). Typically, these hormone concentrations are determined by immunoassay-based methodology illustrating that cross-reaction by T4 might create problems in determination of the concentrations of the other THM if antibodies are not highly specific. Apart from this major issue, the picture intends to illustrate the enormous challenge in reliability of exact determination of free TH concentrations. The magnifying glass already attempts to visualize fT4 concentrations 10-fold expanded compared to the pool of total T4 or 1,000-fold expanded for fT4 and fT3, depicting the reference values for fT4 (>10 pM <23 pM) in the serum of healthy individuals. Even more challenging is this issue in the case of fT3. The 1,000-fold magnification represents the concentration of fT3 (<6.5 pM) in healthy individuals. This presentation is augural in that any interference of (patho)physiology, drugs, or blood components which alter either the huge pool of total T4 of free fraction of fT4 or fT3 poses a tremendous challenge on assay technology and precision in order to provide clinically relevant precise analytic information. The technical and methodological task is even more difficult for the minor THM such as 3,5-T2, 3-T1AM, or Tetrac, Triac and the TH-sulfates discussed above; for the latter group no specific antibodies are yet available. It must be remembered that either changes in TH distribution proteins (TBG, transthyretin, albumin, ApoB100) in the blood (more than 99.7% of TH is protein bound) or variations of concentrations of agents challenging THM binding to any of these blood proteins might markedly affect the readouts of assay systems.

Fig. 3.
Fig. 3.

Schematic presentation of relative thyroid hormone concentrations in human blood. Circles illustrate pool sizes of THM in blood. Symbols for magnifying glasses are used to illustrate the 10- or 1,000-fold magnified circles representing fT4 and fT3 concentrations, respectively. T, total; f, free.

Citation: European Thyroid Journal 8, 3; 10.1159/000497141

Not only are the issues of total versus free THM of relevance but, as discussed above, blood concentrations of the THM do not in many cases reflect local tissue concentrations and thus only demonstrate integral tissue contribution of THM to blood compartments. Sensitivity of hormone determination is a well-known problem not only for THM but also for steroid, protein, and peptide as well as fatty acid-derived hormones because matrix components interfere with both immunological and more advanced MS-based methods of hormone analytics. The major advantage of immunoassay-based hormone analytics is the possibility to directly measure hormone concentration without any preanalytical sample workup, extraction of analyte of interest or dilution of plasma or serum typically used for hormone determination. Nevertheless, technical provisions necessary to adequately and precisely determine free hormone concentrations (ultrafiltration, equilibrium dialysis, etc.) already introduce changes in hormone binding protein interaction and might perturb binding equilibria based on mass action relationships [221]. Determination of fT4 and fT3 concentration (if required at all) has been met with criticism as the introduction of MS methods revealed in the case of fT4 a rather reasonable outcome, but in case of fT3 systematic errors became obvious for most of the immunoassay-based methods [218, 222, 223]. While fT4 determination might provide adequate readout in the ambulatory practice, in hospitalized stationary patients such determinations frequently yield problematic readouts due to interference of underlying disease, medical interventions, or medication. It has also been pointed out that free hormone analytics might provide reasonable precision in the reference range but not in those situations where either high or low free hormone concentrations are encountered. It is a frequent assumption that MS might represent the “gold standard” in hormone analytics as it can precisely determine both quantity and quality of the ligands by their molecular fragmentation patterns. However, it is slightly underestimated that also MS needs careful method establishment, validation, and quality control during application. The sensitivity of contemporary tandem MS is so high that this technology requires preanalytical sample workup either by liquid-liquid or solid-phase extraction procedures which need to be adequately established to assure precision of determination, recovery of analytes of interest, and elimination of matrix effects. Many applications currently using MS analysis either for total or free THM unfortunately do not provide adequate information on the precision of preanalytical sample workup and subsequent quantification of analytes of interest by the mass spectrometer. Frequently, only quick methods with an inadequate sample or analyte separation by liquid (or gas) chromatography are applied, and unequivocal identification of analytes of interest does not take into account the existence of isobaric metabolites having the same mass but compound-specific molecular fragmentation pattern. The precise analysis, e.g. of T3 and rT3 or the three diiodothyronine isomers, requires precise determination of qualifier and quantifier molecular ions and their molecular fragmentation, similar to established MS-based analyses of vitamin D metabolite or several steroid hormone analytical procedures. One great advantage of current MS methodology is the application of the principle of isotope dilution and use of compound-specific stable isotope-labelled internal standards, at least if these are available, which is not the case for all THM of interest. It is not adequate to just use one single isotope-labelled internal standard, e.g. T4, for all THM, which have different physicochemical, extraction, and ionization properties. Implementation of MS in human serum THM analytics still requires intensive method development and validation of assay procedures especially for the minor THM such as 3-T1AM, T2-isomers, or other metabolites. Several reviews recently addressed this issue and discussed the methodology applied for THM analytics in experimental animal models, cell culture research paradigms, and first experience in the application of the MS method for human serum or plasma4 [224, 225]. MS also is applied to the quality control of content and composition of L-T4 medication [226, 227].

Conclusions

Synthesis and metabolism of the classical TH, i.e. T4 mainly considered as “prohormone” and thyromimetically T3, has been studied in detail during the last decades. Development-, tissue-, and cell-specific expression of the three Dio enzymes, activating and inactivating T4 and T3, generates several other THM such as rT3, 3,5-T2, and, in combination with decarboxylases such as ODC, 3-T1AM and its amine oxidation product 3-TA1. These metabolites, identified in human serum and several tissues, exert various TH-like or in part antagonistic actions at least after application of pharmacological doses in animal experimental models. The thyroacetic acid derivatives Tetrac and Triac, also endogenous THM, are of clinical interest in the treatment of TH resistance and AHDS (in the case of Triac) or because they interfere with T4 and T3 rapid effects, mediated via the integrin ανβ5 plasma membrane receptor (in case of Tetrac). Triac binds and activates TR and has the ability to bypass MCT8 in cellular uptake, which is deficient in AHDS. Whether the thyromimetic “hot” actions of exogenous 3,5-T2 are also exerted by endogenous 3,5-T2 production and synthesis still has to be established. Similarly, the function of endogenously formed 3-T1AM and its oxidation product 3-TA1 requires more detailed studies to confirm the variety of metabolic and neurological effects demonstrated after pharmacological administration of these compounds in animal experimental models. Analytics to determine the patterns and profiles of these THM and their precursors need more detailed development, validation, and application of MS-based methods. Apart from the knowledge on classical TH T4 and T3, the role of THM in fine tuning of physiological and pathophysiological conditions related to TH metabolism and action demands more careful consideration of their distinct and specific cellular actions in vivo. The routine analytics of THM in clinical practice still has to be boosted by more intensive interdisciplinary organized research.

Acknowledgements

This project has been funded by the grants from the Deutsche Forschungsgemeinschaft (DFG) priority program 1629 ThyroidTransAct (Ko 922/16-1/2 and 922/17-1/2). The expert secretarial support by Elke Abdel-Karim, IEÉ, during the preparation of this manuscript is greatly acknowledged.

Disclosure Statement

The author declares no conflict of interest related to the subject of this review and as clinically oriented basic scientist neither advises patients nor companies producing thyroid hormone medications. The author has no ethical conflicts to disclose.

References

For references, supplements and supplementary figures, see online supplementary material.

Footnotes

verified

References

  • 1

    Mackenzie HW . A Case of Myxoedema Treated with Great Benefit by Feeding with Fresh Thyroid Glands. Br Med J. 1892 Oct 29;2(1661):940-1. PMID: 20753900 PMCID: PMC2421275

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 2

    Magnus-Levy A . Ueber den respiratorischen Gaswechsel unter dem Einfluss der Thyreoidea sowie unter verschiedenen pathologischen Zuständen. Berliner Klinische Wochenschrift. 1895;29(Juli):650–3.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 3↑

    Hennessey JV . HiSTORICAL AND CURRENT PERSPECTIVE IN THE USE OF THYROID EXTRACTS FOR THE TREATMENT OF HYPOTHYROIDISM. Endocr Pract. 2015 Oct;21(10):1161–70. 1530-891X

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 4↑

    de Carvalho GA , Paz-Filho G, Mesa Junior C, Graf H. Management of endocrine disease: Pitfalls on the replacement therapy for primary and central hypothyroidism in adults. Eur J Endocrinol. 2018 Jun;178(6):R231-R244. doi: 10. EJE-17-0947. RE:view. 1530.0899-1510

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 5

    Michaelsson LF , Medici BB, la Cour JL, Selmer C, Røder M, Perrild H, et al. Treating Hypothyroidism with Thyroxine/Triiodothyronine Combination Therapy in Denmark: Following Guidelines or Following Trends? Eur Thyroid J. 2015 Sep;4(3):174–80. 2235-0640

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 6

    Wiersinga WM . Therapy of endocrine disease: T4 + T3 combination therapy: is there a true effect? Eur J Endocrinol. 2017 Dec;177(6):R287-R296. doi: 10. EJE-17-0645. RE:view. 1530.0899-1510

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 7

    GROSS J . PITT-RIVERS R. 3:5:3′ -triiodothyronine. 1. Isolation from thyroid gland and synthesis. Biochem J. 1953 Mar;53(4):645–50. 0264-6021

  • 8

    GROSS J . PITT-RIVERS R. 3:5:3′-triiodothyronine. 2. Physiological activity. Biochem J. 1953 Mar;53(4):652–7. 0264-6021

  • 9

    Vella KR , Hollenberg AN. The actions of thyroid hormone signaling in the nucleus. Mol Cell Endocrinol. 2017 Dec 15;458:127-135. doi: . Review. PMID: 28286327

  • 10

    Zimmermann MB . Research on iodine deficiency and goiter in the 19th and early 20th centuries. J Nutr. 2008 Nov;138(11):2060–3. 0022-3166

  • 11

    Kendall EC . The Isolation in Crystalline Form of the Compound Containing Iodin, Which Occurs in the Thyroid Its Chemical Nature and Physiologic Activity. JAMA. 1915;64(25):2042–3. 0098-7484

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 12

    Harington CR , Barger G. Chemistry of Thyroxine: Constitution and Synthesis of Thyroxine. Biochem J. 1927;21(1):169–83. 0264-6021

  • 13

    Hird F Jr , Trikojus VM. Paper partition chromatography with thyroxine and analogues. Aust J Sci. 1948 Jun;10(6):185–7.0365-3668

  • 14

    Barker SB , Klitgaard HM. Metabolism of tissues excised from thyroxine-injected rats. Am J Physiol. 1952 Jul;170(1):81–6. 0002-9513

  • 15

    Gudernatsch JF . Feeding experiments on tadpoles. I. The influence of specific organs given as food on growth and differentiation. Arch. F. Entwicklungsmech. D. Org. 1912;35(3):457–83.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 16

    Braverman LE , Ingbar SH, Sterling K. Conversion of thyroxine (T4) to triiodothyronine (T3) in athyreotic human subjects. J Clin Invest. 1970 May;49(5):855–64. 0021-9738

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 17

    Binding of selected iodothyronine analogues to receptor sites of isolated rat hepatic nuclei. High Koerner D, Schwartz HL, Surks MI, Oppenheimer JH. correlation between structural requirements for nuclear binding and biological activity. J Biol Chem. 1975 Aug 25;250(16):6417-23. PMID: 16 242

    • PubMed
    • Export Citation
  • 18

    Sterling K , Milch PO, Brenner MA, Lazarus JH. Thyroid hormone action: the mitochondrial pathway. Science. 1977 Sep;197(4307):996–9. 0036-8075

  • 19

    Wrutniak-Cabello C , Casas F, Cabello G. Mitochondrial T3 receptor and targets. Mol Cell Endocrinol. 2017 Dec;458:112–20. 0303-7207

  • 20

    Sap J , Muñoz A, Damm K, Goldberg Y, Ghysdael J, Leutz A, et al. The c-erb-A protein is a high-affinity receptor for thyroid hormone. Nature. 1986 Dec;324(6098):635–40. 0028-0836

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 21

    Weinberger C , Thompson CC, Ong ES, Lebo R, Gruol DJ, Evans RM. The c-erb-A gene encodes a thyroid hormone receptor. Nature. 1986 Dec;324(6098):641–6. 0028-0836

  • 22

    Köhrle J . Thyroid Hormones and Derivatives: Endogenous Thyroid Hormones and Their Targets. Methods Mol Biol. 2018;1801:85–104. 1064-3745

  • 23

    Louzada RA , Carvalho DP. Similarities and Differences in the Peripheral Actions of Thyroid Hormones and Their Metabolites. Front Endocrinol (Lausanne). 2018 Jul 19;9:394. doi: . eCollection 2018. Review. PMID: 30072951

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 24

    Moreno M , Giacco A, Di Munno C, Goglia F. Direct and rapid effects of 3,5-diiodo-L-thyronine (T2). Mol Cell Endocrinol. 2017 Dec;458:121–6. 0303-7207

  • 25↑

    Jonas W , Lietzow J, Wohlgemuth F, Hoefig CS, Wiedmer P, Schweizer U, et al. 3,5-Diiodo-L-thyronine (3,5-t2) exerts thyromimetic effects on hypothalamus-pituitary-thyroid axis, body composition, and energy metabolism in male diet-induced obese mice. Endocrinology. 2015 Jan;156(1):389–99. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 26

    Padron AS , Neto RA, Pantaleão TU, de Souza dos Santos MC, Araujo RL, de Andrade BM, et al. Administration of 3,5-diiodothyronine (3,5-T2) causes central hypothyroidism and stimulates thyroid-sensitive tissues. J Endocrinol. 2014 Jun;221(3):415–27. 0022-0795

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 27

    Baur A , Bauer K, Jarry H, Köhrle J. 3,5-diiodo-L-thyronine stimulates type 1 5'deiodinase activity in rat anterior pituitaries in vivo and in reaggregate cultures and GH3 cells in vitro. Endocrinology. 1997 Aug;138(8):3242–8. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 28

    Scanlan TS , Suchland KL, Hart ME, Chiellini G, Huang Y, Kruzich PJ, et al. 3-Iodothyronamine is an endogenous and rapid-acting derivative of thyroid hormone. Nat Med. 2004 Jun;10(6):638–42. 1078-8956

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 29

    Hoefig CS , Wuensch T, Rijntjes E, Lehmphul I, Daniel H, Schweizer U, et al. Biosynthesis of 3-Iodothyronamine From T4 in Murine Intestinal Tissue. Endocrinology. 2015 Nov;156(11):4356–64. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 30

    Hoefig CS , Zucchi R, Köhrle J. Thyronamines and Derivatives: Physiological Relevance, Pharmacological Actions, and Future Research Directions. Thyroid. 2016 Dec;26(12):1656-1673. Review. PMID: 27650974

    • PubMed
    • Export Citation
  • 31

    E. Baumann Ueber das normale Vorkommen von Jod im Thierkörper Z Physiol Chem. 1895; 21:319–30

    • PubMed
    • Export Citation
  • 32

    Antonica F , Kasprzyk DF, Opitz R, Iacovino M, Liao XH, Dumitrescu AM, et al. Generation of functional thyroid from embryonic stem cells. Nature. 2012 Nov;491(7422):66–71. 0028-0836

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 33

    Kurmann AA , Serra M, Hawkins F, Rankin SA, Mori M, Astapova I, et al. Regeneration of Thyroid Function by Transplantation of Differentiated Pluripotent Stem Cells. Cell Stem Cell. 2015 Nov;17(5):527–42. 1934-5909

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 34

    Davies TF . Is thyroid transplantation on the distant horizon? Thyroid. 2013 Feb;23(2):139–41. 1050-7256

  • 35

    Nilsson M , Fagman H. Development of the thyroid gland. Development. 2017 Jun 15;144(12):2123-2140. doi: . Review. PMID: 28634271

  • 36

    Ramsden DB , Lawson AM, Raw PJ, Hoffenberg R. The identification of 3,3′, 5,5′,-tetraiodothyroformic acid within the rat liver. Biochem J. 1974 Oct;143(1):47–50. 0264-6021

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 37

    Carvalho DP , Dupuy C. Thyroid hormone biosynthesis and release. Mol Cell Endocrinol. 2017 Dec;458:6–15. 0303-7207

  • 38

    Meinhold H , Gramm HJ, Meissner W, Zimmermann J, Schwander J, Dennhardt R, et al. Elevated serum diiodotyrosine (DIT) in severe infections and sepsis: DIT, a possible new marker of leukocyte activity. J Clin Endocrinol Metab. 1991 Apr;72(4):945–53. 0021-972X

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 39

    Klebanoff SJ , Green WL. Degradation of thyroid hormones by phagocytosing human leukocytes. J Clin Invest. 1973 Jan;52(1):60–72. 0021-9738

  • 40

    Chopra IJ . A radioimmunoassay for measurement of 3,3′,5′-triiodothyronine (reverse T3). J Clin Invest. 1974 Sep;54(3):583–92. 0021-9738

  • 41

    Ködding R , Hesch RD. L-3', 5'-diiodothyronine in human serum. Lancet. 1978 Nov 11;2(8098):1049. PMID: 82062

    • PubMed
    • Export Citation
  • 42

    Burman KD . Recent developments in thyroid hormone metabolism: interpretation and significance of measurements of reverse T3, 3,3'T2, and thyroglobulin. Metabolism. 1978 May;27(5):615–30. 0026-0495

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 43

    Wu SY , Polk DH, Chen WL, Fisher DA, Huang WS, Yee B. A 3,3′-diiodothyronine sulfate cross-reactive compound in serum from pregnant women. J Clin Endocrinol Metab. 1994 Jun;78(6):1505–9.0021-972X

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 44

    Köhrle J. Thyroid hormone deiodinases--a selenoenzyme family acting as gate keepers to thyroid hormone action. Acta Med Austriaca. 1996;23(1-2):17-30. Review. PMID: 8767511

    • PubMed
    • Export Citation
  • 45

    Manna D , Mugesh G. Regioselective deiodination of thyroxine by iodothyronine deiodinase mimics: an unusual mechanistic pathway involving cooperative chalcogen and halogen bonding. J Am Chem Soc. 2012 Mar;134(9):4269–79. 0002-7863

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 46

    Manna D , Mondal S, Mugesh G. Halogen bonding controls the regioselectivity of the deiodination of thyroid hormones and their sulfate analogues. Chemistry. 2015 Feb;21(6):2409–16. 0947-6539

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 47

    Schweizer U , Schlicker C, Braun D, Köhrle J, Steegborn C. Crystal structure of mammalian selenocysteine-dependent iodothyronine deiodinase suggests a peroxiredoxin-like catalytic mechanism. Proc Natl Acad Sci USA. 2014 Jul;111(29):10526–31. 0027-8424

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 48

    Köhrle J. Iodothyronine deiodinases. Methods Enzymol. 2002;347:125-67. Review. PMID: 11898402

  • 49

    Goto K , Sonoda D, Shimada K, Sase S, Kawashima T. Modeling of the 5′-deiodination of thyroxine by iodothyronine deiodinase: chemical corroboration of a selenenyl iodide intermediate. Angew Chem Int Ed Engl. 2010;49(3):545–7. 0570-0833

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 50

    Köhrle J , Jakob F, Contempré B, Dumont JE. Selenium, the thyroid, and the endocrine system. Endocr Rev. 2005 Dec;26(7):944–84. 0163-769X

  • 51

    Fortino M , Marino T, Russo N, Sicilia E. A DFT investigation of a bulky biomimetic model catalyzing the 5′-outer ring deiodination of thyroxine. J Mol Model. 2016 Dec;22(12):287. 1610-2940

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 52

    Doerge DR , Takazawa RS. Mechanism of thyroid peroxidase inhibition by ethylenethiourea. Chem Res Toxicol. 1990 Mar-Apr;3(2):98–101. 0893-228X

  • 53

    Valderrama B , Ayala M, Vazquez-Duhalt R. Suicide inactivation of peroxidases and the challenge of engineering more robust enzymes. Chem Biol. 2002 May;9(5):555-65. Review. PMID: 12031662

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 54

    Horst C , Rokos H, Seitz HJ. Rapid stimulation of hepatic oxygen consumption by 3,5-di-iodo-L-thyronine. Biochem J. 1989 Aug;261(3):945–50. 0264-6021

  • 55

    Moreno M , Lanni A, Lombardi A, Goglia F. How the thyroid controls metabolism in the rat: different roles for triiodothyronine and diiodothyronines. J Physiol. 1997 Dec;505(Pt 2):529–38. 0022-3751

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 56

    Moreno M , Lombardi A, Beneduce L, Silvestri E, Pinna G, Goglia F, et al. Are the effects of T3 on resting metabolic rate in euthyroid rats entirely caused by T3 itself? Endocrinology. 2002 Feb;143(2):504–10. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 57

    Goglia F. Biological effects of 3,5-diiodothyronine (T2). Biochemistry (Mosc). 2005 Feb;70(2):164-72. Review. PMID: 15807655

    • PubMed
    • Export Citation
  • 58

    Burger AG . Is there a physiological role for reverse triiodothyronine? Acta Med Austriaca. 1988;15 Suppl 1:30-3. Review. PMID: 3051832

  • 59

    Van den Berghe G. Non-thyroidal illness in the ICU: a syndrome with different faces. Thyroid. 2014 Oct;24(10):1456-65. doi: . Review. PMID: 24845024

  • 60

    Leonard JL , Koehrle J. Chapter 8: Intracellular Pathways of Iodothyronine Metabolism. In: Braverman LE, Utiger RD, editors. Werner and Ingbar’s The Thyroid: a fundamental and clinical text. 7th ed. Philadelphia, London: Lippincott Williams & Wilkins; 1996. pp. 125–61.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 61

    Farwell AP , Dubord-Tomasetti SA, Pietrzykowski AZ, Stachelek SJ, Leonard JL. Regulation of cerebellar neuronal migration and neurite outgrowth by thyroxine and 3,3′,5′-triiodothyronine. Brain Res Dev Brain Res. 2005 Jan;154(1):121–35. 0165-3806

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 62

    Deng H , Hu H, Fang Y. Multiple tyrosine metabolites are GPR35 agonists. Sci Rep. 2012;2(373):373. 2045-2322

  • 63

    Ambrosio R , De Stefano MA, Di Girolamo D, Salvatore D. Thyroid hormone signaling and deiodinase actions in muscle stem/progenitor cells. Mol Cell Endocrinol. 2017 Dec 25;459:79-83. doi: . Review. PMID: 28630021

    • Crossref
    • PubMed
    • Export Citation
  • 64

    Miro C , Ambrosio R, De Stefano MA, Di Girolamo D, Di Cicco E, Cicatiello AG, et al. The Concerted Action of Type 2 and Type 3 Deiodinases Regulates the Cell Cycle and Survival of Basal Cell Carcinoma Cells. Thyroid. 2017 Apr;27(4):567–76. 1050-7256

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 65

    Hüfner M , Grussendorf M, Lorenz U, Knöpfle M. 3,3′,5′-Triiodothyronine (Reverse T3) in amniotic fluid and cord serum. Eur J Pediatr. 1977 Jul;125(3):213–7. 0340-6199

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 66

    Cettour-Rose P , Visser TJ, Burger AG, Rohner-Jeanrenaud F. Inhibition of pituitary type 2 deiodinase by reverse triiodothyronine does not alter thyroxine-induced inhibition of thyrotropin secretion in hypothyroid rats. Eur J Endocrinol. 2005 Sep;153(3):429–34. 0804-4643

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 67

    Goto-Inoue N , Sato T, Morisasa M, Kashiwagi A, Kashiwagi K, Sugiura Y, et al. Utilizing mass spectrometry imaging to map the thyroid hormones triiodothyronine and thyroxine in Xenopus tropicalis tadpoles. Anal Bioanal Chem. 2018 Feb;410(4):1333–40. 1618-2642

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 68

    Darras VM , Houbrechts AM, Van Herck SL. Intracellular thyroid hormone metabolism as a local regulator of nuclear thyroid hormone receptor-mediated impact on vertebrate development. Biochim Biophys Acta. 2015 Feb;1849(2):130-41. doi: . Review. PMID: 24844179

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 69

    Schiffer L , Arlt W, Storbeck KH. Intracrine androgen biosynthesis, metabolism and action revisited. Mol Cell Endocrinol. 2018 Apr 15;465:4-26. doi: . Review. PMID: 28865807

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 70

    Woods C , Tomlinson JW. The Dehydrogenase Hypothesis. Adv Exp Med Biol. 2015;872:353–80. 0065-2598

  • 71

    Bikle DD . Vitamin D Metabolism, Mechanism of Action, and Clinical Applications. Chem Biol. 2014 Mar 20; 21(3): 319–329. doi: [ PMCID: PMC3968073 PMID: 24529992

    • Crossref
    • PubMed
    • Export Citation
  • 72

    Domingues JT , Cattani D, Cesconetto PA, Nascimento de Almeida BA, Pierozan P, Dos Santos K, et al. Reverse T3 interacts with αvβ3 integrin receptor and restores enzyme activities in the hippocampus of hypothyroid developing rats: insight on signaling mechanisms. Mol Cell Endocrinol. 2018 Jul;470:281–94. 0303-7207

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 73

    Hercbergs A , Mousa SA, Davis PJ. Nonthyroidal Illness Syndrome and Thyroid Hormone Actions at Integrin αvβ3. J Clin Endocrinol Metab. 2018 Apr;103(4):1291–5. 0021-972X

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 74

    Roche J , Michel R, Wolf W. [Probable presence of 3,3',5'-triiodothyronine in thyroglobulin]. C R Hebd Seances Acad Sci. 1955 Jan 10;240(2):251-3. French. PMID: 14352480

    • PubMed
    • Export Citation
  • 75

    Roche J , Michel R, Nunez U, Wolf W. [Two new hormonal constituents of the thyroid gland: 3, 3'-diiodothyronine and 3, 3', 5'-triiodothyronine]. Biochim Biophys Acta. 1955 Sep;18(1):149-50. French. PMID: 13260262

    • PubMed
    • Export Citation
  • 76

    Chopra IJ . Chapter 7: Nature, source and relative significance of circulating thyroid hormones. In: Braverman LE, Utiger RD, editors. Werner and Ingbar’s The Thyroid: a fundamental and clinical text. 7th ed. Philadelphia, London: Lippincott Williams & Wilkins; 1996. pp. 111–24.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 77

    Schmidt RL , LoPresti JS, McDermott MT, Zick SM, Straseski JA. Does Reverse Triiodothyronine Testing Have Clinical Utility? An Analysis of Practice Variation Based on Order Data from a National Reference Laboratory. Thyroid. 2018 Jul;28(7):842–8. 1050-7256

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 78

    Lehmphul I , Brabant G, Wallaschofski H, Ruchala M, Strasburger CJ, Köhrle J, et al. Detection of 3,5-diiodothyronine in sera of patients with altered thyroid status using a new monoclonal antibody-based chemiluminescence immunoassay. Thyroid. 2014 Sep;24(9):1350–60. 1050-7256

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 79

    Pietzner M , Lehmphul I, Friedrich N, Schurmann C, Ittermann T, Dörr M, et al. Translating pharmacological findings from hypothyroid rodents to euthyroid humans: is there a functional role of endogenous 3,5-T2? Thyroid. 2015 Feb;25(2):188–97. 1050-7256

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 80

    Massolt ET , van der Windt M, Korevaar TI, Kam BL, Burger JW, Franssen GJ, et al. Thyroid hormone and its metabolites in relation to quality of life in patients treated for differentiated thyroid cancer. Clin Endocrinol (Oxf). 2016 Nov;85(5):781–8. 0300-0664

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 81

    Hoefig CS , Köhrle J, Brabant G, Dixit K, Yap B, Strasburger CJ, et al. Evidence for extrathyroidal formation of 3-iodothyronamine in humans as provided by a novel monoclonal antibody-based chemiluminescent serum immunoassay. J Clin Endocrinol Metab. 2011 Jun;96(6):1864–72. 0021-972X

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 82

    Engler D , Burger AG. The deiodination of the iodothyronines and of their derivatives in man. Endocr Rev. 1984 Spring;5(2):151-84. Review. PMID: 6376077

  • 83

    Ramsden DB , Raw PJ, Carter PJ, Hoffenberg R. Estimation of tetraiodothyroacetate in human serum. Proc R Soc Med. 1975 Feb;68(2):69–70.0035-9157

  • 84

    Groeneweg S , Peeters RP, Visser TJ, Visser WE. Triiodothyroacetic acid in health and disease. J Endocrinol. 2017 Aug;234(2):R99-R121. doi: 10. JOE-17-0113. RE:view. 1530.0899-1510

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 85

    Dentice M , Salvatore D. Deiodinases: the balance of thyroid hormone: local impact of thyroid hormone inactivation. J Endocrinol. 2011 Jun;209(3):273–82. 0022-0795

  • 86

    Heinen E , Basler M, Herrmann J, Hafner D, Krüskemper HL. Enzyme kinetic and substrate-binding studies of the thyroxine to 3,5,3′-triiodothyronine converting enzyme in the rat liver microsomal fraction. Endocrinology. 1980 Oct;107(4):1198–204. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 87

    LoPresti JS , Anderson KP, Nicoloff JT. Does a hidden pool of reverse triiodothyronine (rT3) production contribute to total thyroxine (T4) disposal in high T4 states in man. J Clin Endocrinol Metab. 1990 May;70(5):1479–84. 0021-972X

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 88

    Köhrle J . [Transfer and metabolism of thyroid gland hormones in the placenta]. Acta Med Austriaca. 1997;24(4):138–43.0303-8173

  • 89

    Segal AW , Garcia RC, Harper AM, Banga JP. Iodination by stimulated human neutrophils. Studies on its stoichiometry, subcellular localization and relevance to microbial killing. Biochem J. 1983 Jan;210(1):215–25. 0264-6021

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 90

    van der Spek AH , Fliers E, Boelen A. Thyroid hormone metabolism in innate immune cells. J Endocrinol. 2017 Feb;232(2):R67-R81. gotoReview. PMID: 27852725

    • PubMed
    • Export Citation
  • 91

    Mitchell AM , Manley SW, Morris JC, Powell KA, Bergert ER, Mortimer RH. Sodium iodide symporter (NIS) gene expression in human placenta. Placenta. 2001 Feb-Mar;22(2-3):256–8. 0143-4004

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 92

    Peeters RP , Visser TJ. Metabolism of Thyroid Hormone. In: De Groot LJ, Chrousos G, Dungan K, Feingold KR, Grossman A, Hershman JM, Koch C, Korbonits M, McLachlan R, New M, Purnell J, Rebar R, Singer F, Vinik A, editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-2017 Jan 1. PMID: 25905401

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 93

    Gomes-Lima C , Burman KD. Reverse T3 or perverse T3? Still puzzling after 40 years. Cleve Clin J Med. 2018 Jun;85(6):450-455. doi: : 29883303

  • 94

    Akturk M , Oruc AS, Danisman N, Erkek S, Buyukkagnici U, Unlu E, et al. Na+/I- symporter and type 3 iodothyronine deiodinase gene expression in amniotic membrane and placenta and its relationship to maternal thyroid hormones. Biol Trace Elem Res. 2013 Sep;154(3):338–44. 0163-4984

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 95

    Huang SA . Physiology and pathophysiology of type 3 deiodinase in humans. Thyroid. 2005 Aug;15(8):875-81. Review. PMID: 16131330

  • 96

    Dentice M , Antonini D, Salvatore D. Type 3 deiodinase and solid tumors: an intriguing pair. Expert Opin Ther Targets. 2013 Nov;17(11):1369–79. 1472-8222

  • 97

    Nishimura K , Takeda M, Yamashita JK, Shiojima I, Toyoda N. Type 3 iodothyronine deiodinase is expressed in human induced pluripotent stem cell derived cardiomyocytes. Life Sci. 2018 Jun;203:276–81. 0024-3205

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 98

    Renko K , Schäche S, Hoefig CS, Welsink T, Schwiebert C, Braun D, et al. An Improved Nonradioactive Screening Method Identifies Genistein and Xanthohumol as Potent Inhibitors of Iodothyronine Deiodinases. Thyroid. 2015 Aug;25(8):962–8. 1050-7256

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 99

    Ciavardelli D , Bellomo M, Crescimanno C, Vella V. Type 3 deiodinase: role in cancer growth, stemness, and metabolism. Front Endocrinol (Lausanne). 2014 Dec 17;5:215. doi: . eCollection 2014. Review. PMID: 25566187

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 100

    Dentice M , Ambrosio R, Damiano V, Sibilio A, Luongo C, Guardiola O, et al. Intracellular inactivation of thyroid hormone is a survival mechanism for muscle stem cell proliferation and lineage progression. Cell Metab. 2014 Dec;20(6):1038–48. 1550-4131

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 101

    Cicatiello AG , Ambrosio R, Dentice M. Thyroid hormone promotes differentiation of colon cancer stem cells. Mol Cell Endocrinol. 2017 Dec 25;459:84-89. doi: .. Review. PMID: 28342853

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 102

    Popławski P , Wiśniewski JR, Rijntjes E, Richards K, Rybicka B, Köhrle J, Piekiełko-Witkowska A. Restoration of type 1 iodothyronine deiodinase expression in renal cancer cells downregulates oncoproteins and affects key metabolic pathways as well as anti-oxidative system. PLoS One. 2017 Dec 22;12(12):e0190179. doi: . eCollection 2017. PMID: 29272308

    • Crossref
    • PubMed
    • Export Citation
  • 103

    Simonides WS , Mulcahey MA, Redout EM, Muller A, Zuidwijk MJ, Visser TJ, et al. Hypoxia-inducible factor induces local thyroid hormone inactivation during hypoxic-ischemic disease in rats. J Clin Invest. 2008 Mar;118(3):975–83. 0021-9738

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 104

    Huang SA , Tu HM, Harney JW, Venihaki M, Butte AJ, Kozakewich HP, Fishman SJ, Larsen PR. Severe hypothyroidism caused by type 3 iodothyronine deiodinase in infantile hemangiomas. N Engl J Med. 2000 Jul 20;343(3):185-9. No abstract available. PMID: 10900278

    • PubMed
    • Export Citation
  • 105

    Roche J , Michel R. Thyroid hormones and iodine metabolism. Annu Rev Biochem. 1954;23(1):481–500. 0066-4154

  • 106

    Roche J , Michel R. Nature, biosynthesis and metabolism of thyroid hormones. Physiol Rev. 1955 Jul;35(3):583–610. 0031-9333

  • 107

    Albright EC , Lardy HA, Larson FC, Tomita K. Enzymatic conversion of thyroxine and triiodothyronine to the corresponding acetic acid analogues. Endocrinology. 1956 Aug;59(2):252–4. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 108

    Larson FC , Tomita K, Albright EC. The deiodination of thyroxine to triiodothyronine by kidney slices of rats with varying thyroid function. Endocrinology. 1955 Sep;57(3):338–44. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 109

    Albright EC , Lardy HA, Larson FC, Tomita K. Enzymatic conversion of thyroxine to tetraiodothyroacetic acid and of triiodothyronine to triiodothyroacetic acid. J Biol Chem. 1957 Jan;224(1):387–97.0021-9258

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 110

    Roche J , Michel R, Tata J. [The nature of iodinated compounds excreted by liver and kidneys after administration of L-thyroxin and L-3,5,3'-triiodothyronine]. Biochim Biophys Acta. 1954 Dec;15(4):500-7. French. PMID: 13230098

    • PubMed
    • Export Citation
  • 111

    ROCHE J . MICHEL R, JOUAN P, WOLF W [Presence of triiodothyroacetic acid in the rat kidney following administration of triiodothyronine] [Article in French]. C R Hebd Seances Acad Sci. 1955 Dec;241(24):1880–2.0001-4036

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 112

    Myant NB . Enterohepatic circulation of thyroxine in humans. Clin Sci. 1956 Nov;15(4):551–5. 0009-9287

  • 113

    Green WL , Ingbar SH. The peripheral metabolism of tri- and tetraiodothyroacetic acids in man. J Clin Endocrinol Metab. 1961 Dec;21(12):1548–65. 0021-972X

  • 114

    Pittman CS , Shimizu T, Burger A, Chambers JB Jr. The nondeiodinative pathways of thyroxine metabolism: 3,5,3′,5-tetraiodothyroacetic acid turnover in normal and fasting human subjects. J Clin Endocrinol Metab. 1980 Apr;50(4):712–6. 0021-972X

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 115

    Davis PJ , Davis FB, Mousa SA, Luidens MK, Lin HY. Membrane receptor for thyroid hormone: physiologic and pharmacologic implications. Annu Rev Pharmacol Toxicol. 2011;51(1):99–115. 0362-1642

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 116

    Sorimachi K , Yasumura Y. High affinity of triiodothyronine (T3) for nonphenolic ring deiodinase and high affinity of tetraiodothyroacetic acid (TETRAC) for phenolic ring deiodinase in cultured monkey hepatocarcinoma cells and in rat liver homogenates. Endocrinol Jpn. 1981 Dec;28(6):775–83. 0013-7219

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 117

    Koehrle J , Auf’mkolk M, Rokos H, Hesch RD, Cody V. Rat liver iodothyronine monodeiodinase. Evaluation of the iodothyronine ligand-binding site. J Biol Chem. 1986 Sep;261(25):11613–22.0021-9258

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 118

    Neumann P , Cody V, Wojtczak A. Ligand binding at the transthyretin dimer-dimer interface: structure of the transthyretin-T4Ac complex at 2.2 Angstrom resolution. Acta Crystallogr D Biol Crystallogr. 2005 Oct;61(Pt 10):1313–9. 0907-4449

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 119

    Groeneweg S , Peeters RP, Visser TJ, Visser WE. Therapeutic applications of thyroid hormone analogues in resistance to thyroid hormone (RTH) syndromes. Mol Cell Endocrinol. 2017 Dec;458:82–90. 0303-7207

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 120

    Horn S , Kersseboom S, Mayerl S, Müller J, Groba C, Trajkovic-Arsic M, et al. Tetrac can replace thyroid hormone during brain development in mouse mutants deficient in the thyroid hormone transporter mct8. Endocrinology. 2013 Feb;154(2):968–79. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 121

    Kersseboom S , Horn S, Visser WE, Chen J, Friesema EC, Vaurs-Barriere C, et al. 2014. In vitro and mouse studies supporting therapeutic utility of triiodothyroacetic acid in MCT8 deficiency. Mol Endocrinol. 1970;28(12):1961e. 0888-8809

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 122

    Bárez-López S , Obregon MJ, Martínez-de-Mena R, Bernal J, Guadaño-Ferraz A, Morte B. Effect of Triiodothyroacetic Acid Treatment in Mct8 Deficiency: A Word of Caution. Thyroid. 2016 May;26(5):618–26. 1050-7256

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 123

    Delbaere J , Vancamp P, Van Herck SL, Bourgeois NM, Green MJ, Wingate RJ, et al. MCT8 deficiency in Purkinje cells disrupts embryonic chicken cerebellar development. J Endocrinol. 2017 Feb;232(2):259–72. 0022-0795

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 124

    Lameloise N , Siegrist-Kaiser C, O’Connell M, Burger A. Differences between the effects of thyroxine and tetraiodothyroacetic acid on TSH suppression and cardiac hypertrophy. Eur J Endocrinol. 2001 Feb;144(2):145–54. 0804-4643

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 125

    Juge-Aubry CE , Morin O, Pernin AT, Liang H, Philippe J, Burger AG. Long-lasting effects of Triac and thyroxine on the control of thyrotropin and hepatic deiodinase type I. Eur J Endocrinol. 1995 Jun;132(6):751–8. 0804-4643

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 126

    Davis PJ , Sudha T, Lin HY, Mousa SA. Thyroid Hormone, Hormone Analogs, and Angiogenesis. Compr Physiol. 2015 Dec 15;6(1):353-62. doi: . Review. PMID: 26756636

    • Crossref
    • PubMed
    • Export Citation
  • 127

    Rajabi M , Sudha T, Darwish NH, Davis PJ, Mousa SA. Synthesis of MR-49, a deiodinated analog of tetraiodothyroacetic acid (tetrac), as a novel pro-angiogenesis modulator. Bioorg Med Chem Lett. 2016 Aug;26(16):4112–6. 0960-894X

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 128

    Schmohl KA , Müller AM, Wechselberger A, Rühland S, Salb N, Schwenk N, et al. Thyroid hormones and tetrac: new regulators of tumour stroma formation via integrin αvβ3. Endocr Relat Cancer. 2015 Dec;22(6):941–52. 1351-0088

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 129

    Klootwijk W , Friesema EC, Visser TJ. A nonselenoprotein from amphioxus deiodinates triac but not T3: is triac the primordial bioactive thyroid hormone? Endocrinology. 2011 Aug;152(8):3259–67. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 130

    Wang P , Liu S, Yang Q, Liu Z, Zhang S. Functional Characterization of Thyrostimulin in Amphioxus Suggests an Ancestral Origin of the TH Signaling Pathway. Endocrinology. 2018 Oct;159(10):3536–48. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 131

    Holzer G , Roux N, Laudet V. Evolution of ligands, receptors and metabolizing enzymes of thyroid signaling. Mol Cell Endocrinol. 2017 Dec 25;459:5-13. doi: . Review. PMID: 28342854

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 132

    Roche J , Michel R, Etling N, Jouan P. Sur le metabolism hepatique de l’acide 3:5:3′-triiodothyroacetique. Comptes Rendus des Séances de la Société de Biologie. 1956;150:1320.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 133

    Martínez L , Nascimento AS, Nunes FM, Phillips K, Aparicio R, Dias SM, et al. Gaining ligand selectivity in thyroid hormone receptors via entropy. Proc Natl Acad Sci USA. 2009 Dec;106(49):20717–22. 0027-8424

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 134

    Zada, D., Tovin, A., Lerer-Goldshtein, T., Appelbaum, L. Pharmacological and BBB-targeted genetic therapies for thyroid hormone-dependent hypomyelination. Dis. Model Mech. 2016; 9 (11), 1339e1348.

    • PubMed
    • Export Citation
  • 135

    Beck-Peccoz P , Piscitelli G, Cattaneo MG, Faglia G 1983. Successful treatment of hyperthyroidism due to nonneoplastic pituitary TSH hypersecretion with 3,5,3’-triiodothyroacetic acid (TRIAC). J. Endocrinol. Invest 6, 217e223.

    • PubMed
    • Export Citation
  • 136

    Beck-Peccoz P , Sartorio A, De Medici C, Grugni G, Morabito F, Faglia G. Dissociated thyromimetic effects of 3, 5, 3′-triiodothyroacetic acid (TRIAC) at the pituitary and peripheral tissue levels. J Endocrinol Invest. 1988 Feb;11(2):113–8. 0391-4097

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 137↑

    Cohen-Lehman J , Charitou MM, Klein I. Tiratricol-induced periodic paralysis: a review of nutraceuticals affecting thyroid function. Endocr Pract. 2011 Jul-Aug;17(4):610–5. 1530-891X

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 138

    Fernando R , Placzek E, Reese EA, Placzek AT, Schwartz S, Trierweiler A, et al. Elevated Serum Tetrac in Graves Disease: Potential Pathogenic Role in Thyroid-Associated Ophthalmopathy. J Clin Endocrinol Metab. 2017 Mar;102(3):776–85. 1945-7197

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 139

    Ball SG , Sokolov J, Chin WW. 3,5-Diiodo-L-thyronine (T2) has selective thyromimetic effects in vivo and in vitro. J Mol Endocrinol. 1997 Oct;19(2):137–47. 0952-5041

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 140

    Lietzow J , Golchert J, Homuth G, Völker U, Jonas W, Köhrle J. 3,5-T2 alters murine genes relevant for xenobiotic, steroid, and thyroid hormone metabolism. J Mol Endocrinol. 2016 May;56(4):311–23. 0952-5041

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 141

    Senese R , de Lange P, Petito G, Moreno M, Goglia F, Lanni A. 3,5-Diiodothyronine: A Novel Thyroid Hormone Metabolite and Potent Modulator of Energy Metabolism. Front Endocrinol (Lausanne). 2018 Jul 25;9:427. doi: . eCollection 2018. Review. PMID: 30090086

    • Crossref
    • PubMed
    • Export Citation
  • 142

    Silvestri E , Lombardi A, Coppola M, Gentile A, Cioffi F, Senese R, et al. Differential Effects of 3,5-Diiodo-L-Thyronine and 3,5,3′-Triiodo-L-Thyronine On Mitochondrial Respiratory Pathways in Liver from Hypothyroid Rats. Cell Physiol Biochem. 2018;47(6):2471–83. 1015-8987

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 143

    Antonelli A , Fallahi P, Ferrari SM, Di Domenicantonio A, Moreno M, Lanni A, et al. 3,5-diiodo-L-thyronine increases resting metabolic rate and reduces body weight without undesirable side effects. J Biol Regul Homeost Agents. 2011 Oct-Dec;25(4):655–60.0393-974X

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 144

    van der Valk F , Hassing C, Visser M, Thakkar P, Mohanan A, Pathak K, Dutt C, Chauthaiwale V, Ackermans M, Nederveen A, Serlie M, Nieuwdorp M, Stroes E. The effect of a diiodothyronine mimetic on insulin sensitivity in male cardiometabolic patients: a double-blind randomized controlled trial. PLoS One. 2014 Feb 21;9(2):e86890. doi: . eCollection 2014. PMID: 24586256

    • Crossref
    • PubMed
    • Export Citation
  • 145

    Cioffi F , Zambad SP, Chhipa L, Senese R, Busiello RA, Tuli D, et al. TRC150094, a novel functional analog of iodothyronines, reduces adiposity by increasing energy expenditure and fatty acid oxidation in rats receiving a high-fat diet. FASEB J. 2010 Sep;24(9):3451–61. 0892-6638

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 146

    Goldberg IJ , Huang LS, Huggins LA, Yu S, Nagareddy PR, Scanlan TS, et al. Thyroid hormone reduces cholesterol via a non-LDL receptor-mediated pathway. Endocrinology. 2012 Nov;153(11):5143–9. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 147

    Vatner DF1 , Snikeris J1, Popov V1, Perry RJ2, Rahimi Y1, Samuel VT3. 3,5 Diiodo-L-Thyronine (T2) Does Not Prevent Hepatic Steatosis or Insulin Resistance in Fat-Fed Sprague Dawley Rats. PLoS One. 2015 Oct 20;10(10):e0140837. doi: . eCollection 2015.

    • Crossref
    • PubMed
    • Export Citation
  • 148

    Angelin B , Kristensen JD, Eriksson M, Carlsson B, Klein I, Olsson AG, et al. Reductions in serum levels of LDL cholesterol, apolipoprotein B, triglycerides and lipoprotein(a) in hypercholesterolaemic patients treated with the liver-selective thyroid hormone receptor agonist eprotirome. J Intern Med. 2015 Mar;277(3):331–42. 0954-6820

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 149

    Kersseboom S , van Gucht AL, van Mullem A, Brigante G, Farina S, Carlsson B, et al. Role of the Bile Acid Transporter SLC10A1 in Liver Targeting of the Lipid-Lowering Thyroid Hormone Analog Eprotirome. Endocrinology. 2017 Oct;158(10):3307–18. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 150

    Sinha RA , Singh BK, Yen PM. Direct effects of thyroid hormones on hepatic lipid metabolism. Nat Rev Endocrinol. 2018 May;14(5):259-269. doi: . Review. PMID: 29472712

  • 151

    Sacripanti G , Nguyen NM, Lorenzini L, Frascarelli S, Saba A, Zucchi R, Ghelardoni S. 3,5-Diiodo-l-Thyronine Increases Glucose Consumption in Cardiomyoblasts Without Affecting the Contractile Performance in Rat Heart. Front Endocrinol (Lausanne). 2018 May 30;9:282. doi: . eCollection 2018. PMID: 29899732

    • Crossref
    • PubMed
    • Export Citation
  • 152

    Moreno M , Silvestri E, De Matteis R, de Lange P, Lombardi A, Glinni D, et al. 3,5-Diiodo-L-thyronine prevents high-fat-diet-induced insulin resistance in rat skeletal muscle through metabolic and structural adaptations. FASEB J. 2011 Oct;25(10):3312–24. 0892-6638

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 153

    Accorroni A , Saponaro F, Zucchi R. Tissue thyroid hormones and thyronamines. Heart Fail Rev. 2016 Jul;21(4):373-90. doi: . Review. PMID: 27115768

  • 154

    Lanni A , Moreno M, Lombardi A, de Lange P, Silvestri E, Ragni M, et al. 3,5-diiodo-L-thyronine powerfully reduces adiposity in rats by increasing the burning of fats. FASEB J. 2005 Sep;19(11):1552–4. 0892-6638

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 155

    Lombardi A , de Lange P, Silvestri E, Busiello RA, Lanni A, Goglia F, et al. 3,5-Diiodo-L-thyronine rapidly enhances mitochondrial fatty acid oxidation rate and thermogenesis in rat skeletal muscle: AMP-activated protein kinase involvement. Am J Physiol Endocrinol Metab. 2009 Mar;296(3):E497–502. 0193-1849

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 156

    Shang G , Gao P, Zhao Z, Chen Q, Jiang T, Zhang N, et al. 3,5-Diiodo-l-thyronine ameliorates diabetic nephropathy in streptozotocin-induced diabetic rats. Biochim Biophys Acta. 2013 May;1832(5):674–84. 0006-3002

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 157

    Pinna G , Brödel O, Visser T, Jeitner A, Grau H, Eravci M, et al. Concentrations of seven iodothyronine metabolites in brain regions and the liver of the adult rat. Endocrinology. 2002 May;143(5):1789–800. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 158

    Kinne A , Wittner M, Wirth EK, Hinz KM, Schülein R, Köhrle J, et al. Involvement of the L-Type Amino Acid Transporter Lat2 in the Transport of 3,3′-Diiodothyronine across the Plasma Membrane. Eur Thyroid J. 2015 Sep;4 Suppl 1:42–50. 2235-0640

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 159

    Hinz KM , Neef D, Rutz C, Furkert J, Köhrle J, Schülein R, et al. Molecular features of the L-type amino acid transporter 2 determine different import and export profiles for thyroid hormones and amino acids. Mol Cell Endocrinol. 2017 Mar;443:163–74. 0303-7207

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 160

    Eravci M , Pinna G, Meinhold H, Baumgartner A. Effects of pharmacological and nonpharmacological treatments on thyroid hormone metabolism and concentrations in rat brain. Endocrinology. 2000 Mar;141(3):1027–40. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 161

    Pinna G , Broedel O, Eravci M, Stoltenburg-Didinger G, Plueckhan H, Fuxius S, et al. Thyroid hormones in the rat amygdala as common targets for antidepressant drugs, mood stabilizers, and sleep deprivation. Biol Psychiatry. 2003 Nov;54(10):1049–59. 0006-3223

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 162

    Broedel O , Eravci M, Fuxius S, Smolarz T, Jeitner A, Grau H, et al. Effects of hyper- and hypothyroidism on thyroid hormone concentrations in regions of the rat brain. Am J Physiol Endocrinol Metab. 2003 Sep;285(3):E470–80. 0193-1849

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 163

    Dietrich JW , Müller P, Schiedat F, Schlömicher M, Strauch J, Chatzitomaris A, et al. Nonthyroidal Illness Syndrome in Cardiac Illness Involves Elevated Concentrations of 3,5-Diiodothyronine and Correlates with Atrial Remodeling. Eur Thyroid J. 2015 Jun;4(2):129–37. 2235-0640

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 164

    Langouche L , Lehmphul I, Perre SV, Köhrle J, Van den Berghe G. Circulating 3-T1AM and 3,5-T2 in Critically Ill Patients: A Cross-Sectional Observational Study. Thyroid. 2016 Dec;26(12):1674–80. 1050-7256

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 165

    Pietzner M , Homuth G, Budde K, Lehmphul I, Völker U, Völzke H, et al. Urine Metabolomics by (1)H-NMR Spectroscopy Indicates Associations between Serum 3,5-T2 Concentrations and Intermediary Metabolism in Euthyroid Humans. Eur Thyroid J. 2015 Sep;4 Suppl 1:92–100. 2235-0640

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 166

    Friedrich N , Pietzner M, Cannet C, Thuesen BH, Hansen T, Wallaschofski H, Grarup N, Skaaby T, Budde K, Pedersen O, Nauck M, Linneberg A. Urinary metabolomics reveals glycemic and coffee associated signatures of thyroid function in two population-based cohorts. PLoS One. 2017 Mar 2;12(3):e0173078. doi: . eCollection 2017. PMID: 28253303

    • Crossref
    • PubMed
    • Export Citation
  • 167

    Pietzner M , Kacprowski T, Friedrich N. Empowering thyroid hormone research in human subjects using OMICs technologies. J Endocrinol. 2018 Jul;238(1):R13-R29. doi: 10. JOE-18-0117. RE:view. 1530.0899-1510

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 168↑

    Orozco A , Lazcano I, Hernández-Puga G, Olvera A. Non-mammalian models reveal the role of alternative ligands for thyroid hormone receptors. Mol Cell Endocrinol. 2017 Dec;459:59–63. 0303-7207

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 169

    Olvera A , Martyniuk CJ, Buisine N, Jiménez-Jacinto V, Sanchez-Flores A, Sachs LM, et al. Differential transcriptome regulation by 3,5-T2 and 3′,3,5-T3 in brain and liver uncovers novel roles for thyroid hormones in tilapia. Sci Rep. 2017 Nov;7(1):15043. 2045-2322

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 170

    Pinna G , Meinhold H, Hiedra L, Thoma R, Hoell T, Gräf KJ, et al. Elevated 3,5-diiodothyronine concentrations in the sera of patients with nonthyroidal illnesses and brain tumors. J Clin Endocrinol Metab. 1997 May;82(5):1535–42. 0021-972X

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 171

    Iannucci LF , Cioffi F, Senese R, Goglia F, Lanni A, Yen PM, et al. Metabolomic analysis shows differential hepatic effects of T2 and T3 in rats after short-term feeding with high fat diet. Sci Rep. 2017 May;7(1):2023. 2045-2322

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 172

    Harder L , Schanze N, Sarsenbayeva A, Kugel F, Köhrle J, Schomburg L, et al. In vivo Effects of Repeated Thyronamine Administration in Male C57BL/6J Mice. Eur Thyroid J. 2018 Jan;7(1):3–12. 2235-0640

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 173

    Gachkar S , Oelkrug R, Martinez-Sanchez N, Rial-Pensado E, Warner A, Hoefig CS, et al. 3-Iodothyronamine Induces Tail Vasodilation Through Central Action in Male Mice. Endocrinology. 2017 Jun;158(6):1977–84. 0013-7227

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 174

    Hoefig CS , Jacobi SF, Warner A, Harder L, Schanze N, Vennström B, et al. 3-Iodothyroacetic acid lacks thermoregulatory and cardiovascular effects in vivo. Br J Pharmacol. 2015 Jul;172(13):3426–33. 0007-1188

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 175

    Köhrle J , Biebermann H. 3-iodothyronamine - a thyroid hormone metabolite with distinct target profiles and mode of action. Endocr Rev. Forthcoming 2019 Jan. 0163-769X

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 176

    Dratman MB . On the mechanism of action of thyroxin, an amino acid analog of tyrosine. J Theor Biol. 1974 Jul;46(1):255–70. 0022-5193

  • 177

    Hoefig CS , Renko K, Piehl S, Scanlan TS, Bertoldi M, Opladen T, et al. Does the aromatic L-amino acid decarboxylase contribute to thyronamine biosynthesis? Mol Cell Endocrinol. 2012 Feb;349(2):195–201. 0303-7207

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 178

    Roy G , Placzek E, Scanlan TS. ApoB-100-containing lipoproteins are major carriers of 3-iodothyronamine in circulation. J Biol Chem. 2012 Jan;287(3):1790–800. 0021-9258

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 179

    Braulke LJ , Klingenspor M, DeBarber A, Tobias SC, Grandy DK, Scanlan TS, et al. 3-Iodothyronamine: a novel hormone controlling the balance between glucose and lipid utilisation. J Comp Physiol B. 2008 Feb;178(2):167–77. 0174-1578

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 180

    Doyle KP , Suchland KL, Ciesielski TM, Lessov NS, Grandy DK, Scanlan TS, et al. Novel thyroxine derivatives, thyronamine and 3-iodothyronamine, induce transient hypothermia and marked neuroprotection against stroke injury. Stroke. 2007 Sep;38(9):2569–76. 0039-2499

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 181

    Regard JB , Kataoka H, Cano DA, Camerer E, Yin L, Zheng YW, et al. Probing cell type-specific functions of Gi in vivo identifies GPCR regulators of insulin secretion. J Clin Invest. 2007 Dec;117(12):4034–43. 0021-9738

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 182

    Klieverik LP , Foppen E, Ackermans MT, Serlie MJ, Sauerwein HP, Scanlan TS, et al. Central effects of thyronamines on glucose metabolism in rats. J Endocrinol. 2009 Jun;201(3):377–86. 0022-0795

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 183

    Dhillo WS , Bewick GA, White NE, Gardiner JV, Thompson EL, Bataveljic A, et al. The thyroid hormone derivative 3-iodothyronamine increases food intake in rodents. Diabetes Obes Metab. 2009 Mar;11(3):251–60. 1462-8902

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 184

    Frascarelli S , Ghelardoni S, Chiellini G, Galli E, Ronca F, Scanlan TS, et al. Cardioprotective effect of 3-iodothyronamine in perfused rat heart subjected to ischemia and reperfusion. Cardiovasc Drugs Ther. 2011 Aug;25(4):307–13. 0920-3206

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 185

    Venditti P , Napolitano G, Di Stefano L, Chiellini G, Zucchi R, Scanlan TS, et al. Effects of the thyroid hormone derivatives 3-iodothyronamine and thyronamine on rat liver oxidative capacity. Mol Cell Endocrinol. 2011 Jul;341(1-2):55–62. 0303-7207

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 186

    Haviland JA , Reiland H, Butz DE, Tonelli M, Porter WP, Zucchi R, et al. NMR-based metabolomics and breath studies show lipid and protein catabolism during low dose chronic T(1)AM treatment. Obesity (Silver Spring). 2013 Dec;21(12):2538–44. 1930-7381

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 187

    Selen Alpergin ES , Bolandnazar Z, Sabatini M, Rogowski M, Chiellini G, Zucchi R, et al. Metabolic profiling reveals reprogramming of lipid metabolic pathways in treatment of polycystic ovary syndrome with 3-iodothyronamine. Physiol Rep. 2017 Jan;5(1):e13097. 2051-817X

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 188

    Assadi-Porter FM , Reiland H, Sabatini M, Lorenzini L, Carnicelli V, Rogowski M, et al. Metabolic Reprogramming by 3-Iodothyronamine (T1AM): A New Perspective to Reverse Obesity through Co-Regulation of Sirtuin 4 and 6 Expression. Int J Mol Sci. 2018 May;19(5):E1535. 1661-6596

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 189

    Manni ME , De Siena G, Saba A, Marchini M, Dicembrini I, Bigagli E, et al. 3-Iodothyronamine: a modulator of the hypothalamus-pancreas-thyroid axes in mice. Br J Pharmacol. 2012 May;166(2):650–8. 0007-1188

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 190

    Dinter J , Mühlhaus J, Jacobi SF, Wienchol CL, Cöster M, Meister J, et al. 3-iodothyronamine differentially modulates α-2A-adrenergic receptor-mediated signaling. J Mol Endocrinol. 2015 Jun;54(3):205–16. 0952-5041

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 191

    Lehmphul I , Hoefig CS, Köhrle J. 3-Iodothyronamine reduces insulin secretion in vitro via a mitochondrial mechanism. Mol Cell Endocrinol. 2018 Jan;460:219–28. 0303-7207

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 192

    Laurino A , Raimondi L. Commentary: Torpor: The Rise and Fall of 3-Monoiodothyronamine from Brain to Gut-From Gut to Brain? Front Endocrinol (Lausanne). 2017 Aug 22;8:206. doi: . eCollection 2017. PMID: 28878736

    • Crossref
    • PubMed
    • Export Citation
  • 193

    Rutigliano G , Zucchi R. Cardiac actions of thyroid hormone metabolites. Mol Cell Endocrinol. 2017 Dec 15;458:76-81. doi: . Review. PMID: 28069537

    • Crossref
    • PubMed
    • Search Google Scholar